I have drawers and chests full of kydex holsters that I've acquired over the last 10 to 15 years. All but one or two of them has the kydex covering up the magazine release.

I have had a magazine eject while carrying with one of those two holsters. That night I took my gun off and realized there was no mag in it. I searched all over until I found it in a vehicle I had been in earlier in the day.  

I bought a Glock 43 last week and started looking for a simple IWB holster for it. Did I miss the ruling that it is now bad to have your magazine release button covered up while holstered?

Any links to decent IWB kydex holsters that cover the mag release.

It happens!  IMO, it's a weak point in the Glock design; but, in the past, every time I've mention this on-line any number of other Glock owner/carriers have strongly disagreed with me.  (OK, so who cares?)  Personally I consider oversized aftermarket Glock magazine releases to be sheer folly; and, one of the principal reasons, 'Why' I won't use a Gen4 Glock is because of the ill-considered larger magazine release; but, hey, apparently that's just me!    

I carry any one of several different Glock pistols around with me everyday.  On three different occasions during the past twelve years, I've looked down to see my, 'once upon a time' formerly properly inserted Glock magazine lying at my feet!  One time I was walking across a concrete pad behind a firing line, bumped something with the toe of my boot, looked down, and there was my Glock magazine!    

Other shooters seem to think this is hilariously funny; but I never have!  The majority of the time I keep another backup magazine on the other side of my belt; so I suspect I'd be able to get through a, 'missing magazine crisis'; but, still, I'd be sure to lose plenty of time!  

Since I started losing an occasional magazine from my EDC Glocks I've become paranoid about anybody bumping into my gun side; and I'm very careful about how I move my gun side elbow.  I've, also, learned to be careful about passing a motor vehicle's diagonal seatbelt strap between my body, and my Glock.  (Yeah, that's happened to me too!)  

One thing you can do to mitigate this problem is to remove the magazine release from the pistol; (Glock recommends that you do this as seldom as possible; probably because the spring is, already, too weak.) and, then, file off the two sharp front corners.  As for your question about finding a decent Kydex holster for a Glock pistol?  Look at Detroit Holster.  Sorry you had to learn the hard way; but, anyway, welcome to, 'the club'!  

There are a couple of reasons holsters typically don't cover the mag release, admin reload and aftermarket variables being the top two.

Any good company would be happy to cover the mag release on a custom holster though.

Never even thought about it, I have several kydex holsters that I rotate between carrying daily, and I don't think any of them cover the mag release. Ive never lost a mag, but can still relase the mag when holstered to reload for a string of fire or check round count in it.

I personally wouldn't want mine covered on my gen 4 Glock, I would think bumping it against your body would be enough to release the mag.

What you want is a Comp-Tac MTAC. I've had several of their holsters and can't remember having a totally covered mag release. Got one a couple of months ago for the G43 and lo and behold, completely covered mag release.

The MTAC is definitely a good holster. I've ordered four and currently own two.

I was looking for something a little sleeker for my 43 though. I would like to get the infidel model but it doesn't cover the mag release. It's actually the model holster I was wearing when I lost the magazine meantioned in my OP.

Sleeker?

When I press a kydex holster, I've never had my press eject a magazine. I doubt a holster that covers the mag release has the potential to eject a mag from bumping it. I don't understand the need for open mag releases, but sometimes  (rarely) people ask for it opened.

This.
I make all mine open unless otherwise requested.

I have made anout 2 dozen kydex holsters now just as a hobbie and from my experience covering the release on an inner waist model will promote the release of the mag when sitting . I dont cover the mag release anymore but its a personal choice and your expeirence may vary