Mention in changelog that universal notifiers are not yet supported on BSD

CHANGELOG.rst

@@ -102,6 +102,7 @@ Deprecations and removed features
   If this happens, you can use the ``reset`` command from ``ncurses`` to restore the terminal state.
 - ``fish_key_reader --verbose`` is now ignored, so it no longer shows raw byte values or timing information.
   Raw byte values should no longer be necessary because fish now decodes them to the new human-readable key names for builtin bind.
+- Instant propagation of universal variables now only works on Linux and macOS. On other platforms, changes to universal variables may only become visible on the next prompt.
 
 Scripting improvements
 ----------------------

tests/checks/tmux-prompt.fish

@@ -1,5 +1,6 @@
 #RUN: %fish %s
 #REQUIRES: command -v tmux
+#REQUIRES: uname | grep -v BSD
 
 set -g isolated_tmux_fish_extra_args -C '
     function fish_prompt