Sure, all these IDEs are nice, but sometimes you still have to break out a text editor and edit some files.
Case in point, I don't run the Xcode simulator much because it makes my MacBook gasp and wheeze, even the one I bought last summer (just before the M1 Macs came out, dag nab it). So I only just recently noticed one of my projects won't build for the simulator.
After slithering through various proposed Xcode contortions on Stack Overflow, I figured out that the Valid Archs setting has to be removed from Xcode 12, and the most direct way to do it is go into the pbxproj file with a text editor and remove all instances of it.
Ta da! (JIRA should have a "ta da!" resolution). And Apple, thanks for ephemerally documenting this. That's one way of deprecating someething, just pretend it never existed.