Here's the wiki entry about long values in general:
https://wiki.openstreetmap.org/wiki/Multiple_values#Numbered_suffix_in_key

You can always check Taginfo for specific tag:
https://taginfo.openstreetmap.org/keys/addr:flats#similar
If you check e.g. addr:flats:1 you see that is in use since 2015, although this is not very common for obvious reasons. When looking at other tags, you'll find many without the colon - but as described in the Wiki these are mostly generated by old imports and where never properly discussed and documented.

I don't think there is any way to shorten the values, the numbers I see are just too diverse.