Lots of code has been broken because people have written UB code and then optimzers become smarter and vreak the code.

This category of we call ”usless UB” are things that wont help optimizeayions, and all major compiler have to support beacause if they dont, their users complain too much, can be relied on.

Unfortunately its very hard for the average use to know what UB can be relied on (there isnt much).