Problem/Motivation
Just tried to set a taxonomy term field to non-required in the field UI, and the field persisted as required in the form.
This is because forum_form_node_form_alter() forces the widget to required regardless of the setting. The comment says for 'proper' forum modes, but this occurs for any node type using that vocabulary. The only way I was able to find out what this was happening was because I guessed there'd be an alter in forum module forcing the behaviour, but that was after a few minutes head scratching.
Proposed resolution
Remove the line setting the vocabulary to required to allow the field UI to work as expected.
Remaining tasks
Tagging novice because this should be a one-line patch.