The current search features of UBB.threads allows locating posts based off their content, using single and multiple search terms. The search can locate a post from a single user and/or keyword/keywords within the post subject and/or post body, and all searchable by date. And/or selected forum/forums and/or category/categories. (simply click "Search" from the top menu, to access it)
I don't feel there is a need to add additional "post tags" to duplicate the content which is already written within each post. Content that is already searchable.
Adding "post tags" would be a step backwards and will require additional code, queries, tables/columns. Each of which generally have tendencies to slow down forums which rely on running those tasks multiple times in quick succession. Especially adding a poor version of what's already included with the forum software.
If a user wants to include terms specific to the topic/content of their post which makes that single post search able for that specific term/terms, why not use a unique #hashtag to add them https://en.wikipedia.org/wiki/Hashtag