whereused --historical
Does not check the reflog, but otherwise works.
It's possible for it to display something that is not an annexed file,
if a non-annexed file somehow ends up containing something that looks
like the key's name. This seems very unlikely to happen, and it would
add a lot of complexity to detect it and somehow skip over that file,
since the git log would need to either be run again, or not limited to 1
result and canceled once enough results have been read.
Also, it kind of seems ok, if a file refers to a key, to consider that
as a place the key was used, for some definition of used. So, I punted
on dealing with that. May revisit later.
Sponsored-by: Brock Spratlen on Patreon