xinstall is missing
This is the only util I found by now that is used by every other one but somehow is not presented here
IIRC, xinstall is surprisingly tricky and I didn't have the time to un-trickify it.
But yes, it should be revisited.
Perhaps you could look at sbase's implementation? This is allegedly portable.
Except that "baseutils is a collection of Unix userland utilities from OpenBSD." Unless there is a really good reason to do otherwise, this repo should have OpenBSD utilities. The only exception so far has been bc and dc, since those require libressl and it's not worth shipping that in a self-contained repo.
If you don't see it here, it means I haven't had the time to do it. I do take patches.
I think @periish meant inspiration or so, not literally taking xinstall from sbase