Forum tags is something I'd love to see.
Does not matter if it's predefined tags only or not.
That way we could tag stuff with tags like these...
OS Tags: Windows, Linux, Mac
Version Tags: 5.11, 5.20
Beta Tags: 5.20 B16, 5.20 B17
Source Tags: Complete, Include, Snippet
Post Tags: Question, Answer
Special Tags: Bug, Fixed
So a topic/post could have the tags...
Windows, 5.20 B16, Question
and a post following that might have
Windows, 5.20 B16, Answer, Snippet
Now tags on their own are meaningless unless you can see or search for them.
I have no idea how much (or little) work it would be to add a tag module to phpBB3
There should be some topic tag mods available. Or they may be called Topic Prefix mods.
A topic prefix mod depending on the implementation might be backwards/forwards compatible it actually makes use of the topic field in the database. Although there is a topic length limitation on the forum that would need to be increased (should be changeable in the forum admin panel someplace).
This is pure speculation, but I would not be surprised if a future phpBB4 release includes tag/topic prefix support in some way.
For phpBB3 there seems to be this one https://www.phpbb.com/customise/db/mod/tmm (it includes two mods and by the looks of it one can install just the topic prefix one).