@trwnh In Plume you can give tags to articles. They do not refer to a specific part of the text, but are still there. They use type hashtag.
Example: curl "https://blog.ilja.space/~/B/Pattern%20matching%20in%20Elixir%20and%20why%20it's%20so%20great" -H 'accept: application/activity+json' | jq .tag
[ { "href": "https://blog.ilja.space/tag/Elixir", "name": "Elixir", "type": "Hashtag" }, { "href": "https://blog.ilja.space/tag/PatternMatching", "name": "PatternMatching", "type": "Hashtag" } ]