This is not a discussion of hoe to conceal your gun with less on, but is about sweat.
I switched from a Shield .40 to a 9mm 1911 Officers. The reason why was my finding the magazine release stuck, and being unable to eject the magazine.  This happened, not once, but twice.

Now, after a couple sweaty days outside, I am noticing that my 1911 compact slide release starting to shoe sings on rust.  I am not happy about it snot at all. I am carrying it in a minotaur MTAC holster.

I am thinking of a a Stealthgear Onyx holster.  Will that help this problem?

I carried my XDs .45 in a Nate2Tactical holster in Texas in June, with no issues with sweat on the gun. ymmv.

A lot of the hybrid holsters with the "combat cut" leave the magazine release uncovered and some even leave the slide lock/release uncovered. I am fairly certain by the looks of the pictures and reviews that the Onyx will cover your slide release, but your mag release will still be exposed. One possible fix is tucking in an undershirt between you and the gun.

As for coverage/protection of the gun, you'll need to cover the whole thing as one person pointed out with their n82. My guns carried in n82 in the heat with sweat are never wet after a day of use. But that comes at a price, it's harder to draw the weapon. Also, there is a lot of bulk with the N82! It does work though.

If you live where it is hot and humid, rust or corrosion will pretty much be a fact of life unless you are anal about maintenance. Even stainless steel is stainLESS, not stainFREE. The summer after I got it, I rusted and pitted a S&W M649 Bodyguard in just a couple days because I sweated on it like a pig working outside all day and forgot to wipe it down.

The only two guns I've carried that I never had rust issues with are my Glock 26 (older gun from when they were first introduced and Tenifered, not the newer "nitride") and my Ruger LCP. The only reason the LCP hasn't rusted is because I rarely carry it and keep a good coat of Johnson's Paste Wax on the metal parts and magazines precisely because it is blued. I have seen a guy on my PD that managed to rust his G26 that is about the same age as mine because he carried it for years and never maintained it.

I have had the same problem with my Shield that you had, the mag release getting hard to press. The metal in the frame rusted. I didn't know they were a MIM part and figured it was Melonited, evidently MIM can't be Melonited. I took the mag release out, steel wooled the rust off, coated it liberally in BreakFree and haven't had a problem since, as long as I remember to take it out and coat it in BreakFree occasionally. Since this summer has turned out to be hot as hell and I've been sweating like a pig, I've made it a habit to wipe all of my carry/duty guns down with a silicone cloth before I go to bed.

All it takes is a little preventative maintenance to keep your gun pretty much corrosion free. People carried blued guns for years and, while there are some really pitted blued guns, there are also a lot of blued guns that have little or no corrosion. Keep the thing wiped down and you should be OK.

Bub75

Your own chemistry may be at work as well. Some people are naturally more corrosive than others.
 

Bub75 is spot on, all counts.

Rust is a fact of life.

My old stainless 1911 would get spotted; Glocks (old as well) never.

Always wipe 'em down.

Central FL guy - year round we are hot as hell and crazy high humidity. I've been CCW'ing Glocks for over 10 years and have never had a problem, not a spot of rust or anything. I dont have to stay on top of upkeep and lubing, just every so often or after the range. They just dont rust and dont fail, at least not in my situation. Even on long weekend camping or hunting trips, getting drenched in down pours day after day. Flawless.

Central Florida here as well, and I sweat like crazy.  I've had a few mag releases rust on carry guns in years past.  My advice find a gun with a polymer mag release.  BTW, none of the Glocks that I currently carry have had any rust issues.

One thing I have found are people who do not treat the leather of their holsters after sweating a lot tend to show corrosion or rust much faster.  The leather retains the salts you sweat out.

After I am done wearing my holster after sweating a lot I first wipe down the leather with a wet (water) cloth to clean surface leather.  You do not have to soak the leather by any means but wipe it all down.  Second the same stuff I use to treat the leather in my car I use on the leather of my holster.

Leather is skin of an animal just like your skin you must moisturize the leather and clean it from time to time.

Sweating in the Texas heat is the reason I use a kydex holster instead of leather.
Just run in under a faucet when I take it off and throw it on the counter to dry.
I also keep a can of WD-40 in my home office where I usually take my gun off and if its wet I give it a quick blast and wipe it down.

Haven't had an issue carrying my Glocks in over 6 years of carrying.

Me either...south FLA,  But i recommend to ALWAYS wear an undershirt. You need something to suck up the sweat.  The drawback is i buy a 6 pack of undershirts every 4 months. They turn yellowish

I have a Comp-Tac CTAC, it is full Kydex. I wear it on bare skin at 12:00 position everyday all day in a steel shed with no air movement in 100 degree temps so there is a ton of sweat flowing through the holster and gun. Been doing this for years I may clean the gun once or twice a year(fire it often though) with no issues other than surface rust but it is a Glock(fewer metal parts).

There were some places inside the holster that would hold sweat against the gun, I just relieved those with a Dremel, that is why full kydex is superior to leather and leather/kydex hybrids, you can mold or remove material very easily.

Get yourself a couple of Sentry Tuf Cloth's from Midway, Brownell's etc...
and use them on a regular basis.
Obviously you still need to clean and lube the internals but these cloths do
work very well for sweat protection and they're not messy, like coating with oil.
They come in resealable bags and last quite a while.

I used kydex OWB for years but about 4 months ago bit the bullet and bought a StealthFear Onyx IWB. I live in southern GA and it's hot as crap and humid all the time. I have had zero issues with corrosion and I don't carry a glock. It was a little bit of a stretch for me to pay so much for this holster but it's been soooooooo worth it. Buy it, you can't go wrong with the Onyx. it's ridiculously comfortable.

This. I sometimes carry a 5" 1911, but if it gets up over 75-80 degrees, I'm probably sweating. Then it's a G23 in a kydex holster. I have covered that rig in sweat on multiple occasions to the point where you could ring my shirt out. The Glock just thinks it's extra lube.

Of all the lcp's i know have been carried in FL including mine. Ive never heard of 1 rusting.

Me three. The G23 and G19 get carried a lot and no problems, but I wipe em down with a TriFlow soaked red rag every night before they go to bed.

If it's very hot and humid I carry OWB.

But I still carry IWB on hot humid days where I can't carry OWB (out for dinner nice restaurant etc) or even I just don't want to open carry. No sweat on my gun.  Get kydex.  Stay away from leather. Leather absorbs moisture.

Comptac ctac. Mtac creates more sweat due to larger areas of leather

South FL
My experience as well.

The only guns I didn't have a problem with rust in Houston and Corpus were Beretta 96G (Bruniton), BHP (matte blue), Glock (3 diff models), SigPro (Nitron)... I currently carry an XD which I can sweat onto until it runs out the mag well and won't corrode.  I sometimes carry Turkish CZ clone in a Bianchi 100 professional which has something to similar to CZ polycoat.  I've never seen that holster soak through.  When I was a patrolman I used B-C Sheath but I guess they don't make it anymore, but they replaced it with Barricade?  It worked well on a blued 1911 I carried for a while also but I still had rust issues under the grips which was even a problem with stainless guns in Houston humidity due to inability of stainless to hold any lube on the surface. Problem I found is that leather or fabric holsters with a moisture barrier create another issue with holding moisture in or against the surface of the gun.  The holsters I use now completely soak through sometimes, but I weekly wipe them down or after I take them off if I notice they're wet.

Yes.

I rusted the mag catch shut on my Shield, too, OP. Now I carry a G43 and don't worry about it. There is no bolster which will completely shield your pistol from sweat.