The user_applications_dir method currently lacks a site_applications_dir counterpart.

Some suggestions:

• Unix:
  • /usr/share/applications?
  • Or a /usr/local/share/applications:/usr/share/applications multipath? As with site_log_dir (Add site_state_dir() counterpart for user_state_dir() #214) and site_bin_dir (Add site_bin_dir counterpart for user_bin_dir #434), platformdirs might choose not to use a multipath for consistency reasons.
  • Or maybe even use $XDG_DATA_DIRS with appended …/applications as multipath? However, note that this env variable often includes paths of other software, e.g. $HOME/.local/share/flatpak/exports/share:/var/lib/flatpak/exports/share from Flatpak, so platformdirs would need to filter that somehow… That's rather inconvenient.
• Android: user_applications_dir alias (which is an alias for user_data_dir)? As with any other site_*_dir counterpart…
• Windows: %PROGRAMDATA%\Microsoft\Windows\Start Menu\Programs
• Mac OS: /Applications? Sounds about right, but I have no Mac, so no idea…

Note that user_applications_dir currently doesn't accept $appauthor, $appname, $version.

If implemented, user_applications_dir should support the use_site_for_root option and redirect calls to site_applications_dir if enabled.