I happened to have a Spigen iPhone X/XS/11 Pro glass screen protector so I have overlaid it on an iPhone 12 Pro to provide the attached image. You will note that the screen protecter successfully covers the majority of the screen but does miss some of the usable screen area.
If you position it at the top, there's a gap at the bottom of a noticeable amount. Left to right, there's a slight increase in usable space, but it is less noticeable.
My takeaway is that I would alter my earlier comment and say that though an iPhone XS/X/11 Pro will technically fit and work OK if the screen protector is centered in all directions, it is certainly not ideal, and I would not recommend that anyone buy an old screen protector just to save money.
I hope this is helpful to the OP and others.