[moosecombinator]

Author here. The PCOMMIT encoding you mention produces an undefined opcode fault on all existing CPUs (written as #UD in the specs). Since no CPU has shipped supporting PCOMMIT, this gives us high confidence that no code containing PCOMMIT is currently in-use or it would be producing #UD faults. Since the need for PCOMMIT has gone away, that fact allowed us to remove the instruction before it created legacy code we'd be signed up to support forever.

I love your examples of other removals -- didn't know about those.