> why is `close` in libpthread.so?

That's because close() is a pthreads "cancellation point" (see https://man7.org/linux/man-pages/man7/pthreads.7.html for details), so it needs special handling when the process is using pthreads. If the process does not link to libpthread.so, the implementation in libc.so (which probably doesn't have cancellation point support) will be used.

I believe that a few libc functions are reimplemented in libpthread, the idea being that if you don’t link to pthreads, you don’t need the overhead (locking, etc.) that is needed in multithreaded situations. Feels a bit antiquated now…

As for why close specifically though, that’s a good question. I wonder if it has something to do with special libc treatment of the standard fds or anything like that.