I don't know if I would call anyone unfortunate enough to not know about the limitations of select() deserving of such limitations. At least on my Linux box the man page does have a warning at the top of the description section but it is easy to accidentally skim over.

It would be interesting to do something like avoid defining that symbol by default, require `-DENABLE_OBSOLETE_SELECT_API` to make it available. It would cause trouble for compiling old software but it is easy to remedy and at least makes new users extra aware that they shouldn't start using this function.