I’m probably stating the obvious, but to find poplar keys used in Estonia, you could use Geofabrik’s taginfo instead of writing queries yourself or filtering through global data.
I can chime in along CityIdx. It was tag introduced by early phase of Verbatium’s import in 2008. He added that key to some, but not all buildings imported in western part of Tallinn to presumably make OSM map data compatible with Garmin devices popular in Estonia back then. Instead of just deleting the tag, please make sure all of those buildings have correct addr:city tags, as currently most of them seem to be lacking them. In 2020 i ran import which removed some instances of the tag, but not all, because those buildings’ (geometry) didn’t meet certain requirements I can’t recall any more. (link to relevant code snippet)
Tln_ID tag has gone through mailing list suggesting that’s also leftover from early Verbatium imports. I think it’s safe to remove, afaik nowadays transit data relies on peatus.ee’s GTFSs.