I think the behavior in the GNU clone is more consistent with "we added line numbers to the concatenation tool" while whoever added it to OpenBSD's ancestry was probably more concerned with generating listings for printing.

Personally, I'd remove even the POSIX [-u] flag from all cat versions because all of them are un-UNIX-like.

Some GNU tools are particularly ridiculous with the Easter egg features I only know about because LLMs use them all the time —and I have been using Linux for 30 years.