"GNU grep implemented a change that breaks pre-existing scripts using a 46 year old API, but it's OK because the required workaround works everywhere but Solaris 10" seems like not a great statement of engineering design to me.
"GNU grep added a warning to inform you of the deprecation which happened 28 years ago, but only to stderr, and still works like you expect", does to me.
Meh. Look, it broke code. "Still works like you expect" is 100% false.
The deprecation argument is at least... arguable. It was indeed retired from POSIX. But needless deprecation is itself a smell in a situation where you can't audit all the code that uses it. Don't do that. It breaks stuff. It broke the updates in the linked article too. If you have an API, leave it there absent extremely strong arguments for its removal.
