This reasoning sounds quite arbitrary to me. Many smaller roads with much less traffic also have bus routes. Also what degree of transit traffic is necessary so that the road has to "stand out"?

General convention followed elsewhere outside of cities, and as as currently documented[1], is to use highway=secondary for roads of type "tugimaantee" and highway=tertiary for roads of type "kõrvalmaantee". The way this road stands out is now inconsistent to how other roads of these types stand out. I think it should be restored.

[1] https://wiki.openstreetmap.org/wiki/International_highway_classification_equivalence