Bash's posix mode still has a lot of bashisms. The only way to test true posix compliance is to test with other shells like dash and ash. I have found this is the case a lot of time. People will say it's POSIX compliant shell, but it'll obviously only be tested in bash Like at that point just make it a bash script, since pretty much every system under the sun has it.

I was thinking about bash scripts that use bash specific builtins. Some of those require >= x version.