>and some may still rely on self-modification in unknown ways
To the point that they have the EX instruction. It's an indirect logical or and execute single instruction to allow for re enterent self modifying code. How can you even begin to W^X given that you have that instruction?
I don't think EX typically accesses writable memory, so that wouldn't be a big problem (and hardware could actually enforce that target of EX is in executable storage). It's used as a workaround to have variable portion of instructions that aren't normally variable.
The actual non-reentrant code is becoming less common, but it still happens sometimes.
Although I think it could be possible to implement that writable pages cannot be executed via a special bit in PSW. Not sure about practical implications to z/OS, though.
To the point that they have the EX instruction. It's an indirect logical or and execute single instruction to allow for re enterent self modifying code. How can you even begin to W^X given that you have that instruction?