I think he means POSIX. Didn’t check but in some cases posix only covers some options a tool provides not all. It’s a hard lesson I learned while keeping shell scripts portable between Linux and macOS.
The only place that that doesn't support 'grep -E' and 'grep -F' nowadays is Solaris 10. But if you are still using that you will certainly run into many other missing options.
[1] https://pubs.opengroup.org/onlinepubs/007908775/xcu/egrep.ht... [2] https://pubs.opengroup.org/onlinepubs/007908775/xcu/fgrep.ht...
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.