A G21 would be the last Glock to KB. 45acp pressures are ~ 18K and the frame is built to house the 10mm pressures also. It's a real low pressure round, and only a double charge would blow it up.
I had a G21 for many years and shot thousands of lead boolits through it with no KB. A healthy dose of common sense (uh-oh) must be applied. Proper sizing of boolits, NOT shooting jacketed rounds AFTER the lead to clean out the lead, and regular thorough cleanings.
My experience with the G21s chamber is that it was tight (vs my 1911). My lead boolit reloads that would choke my G21 would run through my 1911 like butter.