Did this ever go anywhere? I also found this topic
https://meta.discourse.org/t/add-user-fields-support-for-field-type-tag/42721/5
which also seems to have gone only half-way to a permanent solution.
What I would find useful is a multiple choice dropdown field like in the composer
It would then make sense to be able to search for users with a specific tag.
Currently Customize > User Fields only allows single choice dropdowns (as does the Custom Wizard Plugin) and I’m not sure if searching for those fields returns anything.