Topic prefixes are not topic tags, they are pre-defined categorizing terms and should be a few 5-10 or up to 20.

If you let your users create prefixes, they'll create hundreds of prefixes. You'll not be able to use the prefix drop-down because of the big number of items. Instead of prefixes, your users can create tags for each topic, so the tags are the alternative to the topic prefixes for regular users.

There will never be an option to let users create prefixes. It's like allowing users to create forums.

That's called "Topic Prefix", I changed the topic title. This feature comes with wpForo Topic Prefix and Tags Manager addon, this is the only addon we use here at wpForo support forum.