In addition to multipurpose tag ref and some global ones like clc:id, uic_ref and gtfs_id I did find some interesting tag keys:
tln_ID - bus stops, looks like it’s from historic import. Is it still relevant? The source still exists in some form but the use of this tag is declining.
number - used for seamarks and this is linked to Märgi number - source.
muuseum_id - source has new webpage and this 13 year old id don’t seem to be relevant. Should it be removed?
sadamaregister:ref - along with: sadamaregister:url, sadamaregister:code and ref:LOCODE This is recent addition by @jemm who has been mapping marinas. Great work, but I’m not sure if all 4 tags are really necessary since some information is shared between them. Not an issue, but just something I noticed.
bridge:ref - global tag. Linked to Silla number in Teeregister.
CityIdx - old relic, quite confident this can be deleted.
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.
CityIdx, yep, looks like some of these building are missing addr:city. And since there are only 32 instances it’s very easy to do additional manual checks to see if the building still exists or even add the maaamet:ETAK tag.