I currently planning a Form 1 suppressor build for on my GSG 1911-22. How safe would it be to notch the slide to where the slide stop would lock the slide to prevent cycling for a quieter shot? FYI, I would never consider doing this on a 45, this isn't me beating around the bush to not look crazy.

You will probably get better feed back from the suppressor forum.

A buddy of mine used his thumb to hold back the slide on his High Standard. Yes... that sounds  A BIT WIERD.
His theory was; if the slide did not start moving to does not have much energy. Will be tougher to do on a 1911 type due to the hammer. You could try a gloved heel of palm on the slide to test this (kinda sounds like a dare but it worked for the High Standard).

Just don't try that with anything else that a .22.

Just hold your thumb up to the slide when you fire, a 22 does not have that much power, I do that with a Ruger 22/45 lite and it works fine.

TBA suppressors makes a slide lock for ruger MK and 22/45 pistols.

H&K tried doing the slide lock on the Mark 23 offensive pistol prototypes. Seemed like a good idea for a military weapon. For whatever reason, the production version didn't have it.

That is because it made/makes more noise to cycle the action after the shot than having the weapon cycle normally (two sounds; one very distinctly weapon like).