An excerpt from NEWS.23.1:
** The following platforms will be removed in a future Emacs version:Plus some others that I haven't used.
If you are still using Emacs on one of these platforms, please email
firstname.lastname@example.org to inform the Emacs developers.
*** Old GNU/Linux systems based on libc version 5.
*** Old FreeBSD, NetBSD, and OpenBSD systems based on the COFF
*** Solaris versions 2.6 and below.