It's just not possible to inform each and every user whose tags I change. In this changeset alone this would mean to check 14 objects for their history to figure out who added which tag in which changeset I have to leave a comment on.

"compacted:material" won't solve the issue - the actual surface material will be hidden for all tools that are not aware of your new and undocumented tag. 
If we follow your line of thought the actual tagging would need to be
surface=paved
paved=compacted
compacted=limestone
... which clearly is not useful.

Key:surface explicitly allows for custom values, and this is in use in various states and countries.
