tmux does not include support for searching for its config file(s) anywhere other than ~/.tmux.conf. Support to search $XDG_CONFIG_HOME should be added. Upstream has opted not to support the XDG Base Directory Specification[1][2] instead telling users to symlink tmux.conf which effectively defeats part of the goal of the specification.

In the years since the introduction of the XDG specifications it has seen wide and increasing adoption across a variety of environments. I would argue that the community has reached a de facto consensus and that support for it should be adopted unless such support would badly break backward compatibility or potentially lead to serious security issues.

Further, adding support to tmux for searching $XDG_CONFIG_HOME in no way requires a user to follow the XDG spec; if anyone wishes to continue placing their config files in the root of their home directory, they are free to do so.

[1] https://github.com/tmux/tmux/issues/142
[2] http://thread.gmane.org/gmane.comp.terminal-emulators.tmux.user/6013