Am Montag, 12. April 2021, 22:09:51 CET schrieb sergey.v.liverko@yandex.ru:
it would even simplify tagging for signals in countries where different heights at the same pole are permitted
It also allows to simplify initial mapping of signal - you don't need to know exact type of signal in order to add its height to OSM (for example, if you've seen it from backward only).
I finally put that into a wiki page:
https://wiki.openstreetmap.org/wiki/Proposed_features/railway:signal:height
This is not finished yet, so help is welcome.