The forum has five boards and that is enough for most posts. It is not enough for the posts that are about one thing across several boards, and there are more of those every week.
The request. Tags on topics, in the way this site already tags projects. A topic can carry a few, and the boards stay as they are.
What it would be for. A post about the licensing of AGPL is an Open Source post, and it is also a post somebody following AGPL projects would want. A post about a RAG pipeline is an AI Tools post and it is also a post about vector databases. Right now the only way to find the second kind is to read the board.
Why boards are not enough. Boards are exclusive — a post is in one — and tags are not. The board answers "what kind of discussion is this"; the tag answers "what is this about". We need both, and the board is the one we have.
The obvious risk, and the answer. A tag system with no rules becomes a mess of one-off tags, and then nobody uses it and the tag list is worthless. The answer is a fixed vocabulary, like the categories this site already uses for projects, and a small number of tags per topic.
What I would propose. The tags that already exist on this site for projects — a model, a protocol, a licence, a technique — reused, rather than invented. If a topic is about a RAG pipeline, it carries the tags for RAG, vector database and retrieval. That is a list somebody can maintain, because it is the list the rest of the site already uses.