Okay, so today I first came across the word relevancy in written. I grasped its meaning without problem, checking in a dictionary that my hunch was right.
So it would appear that relevance and relevancy both mean the same. They only differ in one letter and both are nouns (i.e. there is no distinction like adjective vs. adverb here).
All that said: what are the actual differences, however subtle, in the meaning of the two words? E.g. in what situations would I use one but couldn't use the other? Is one or the other used only in particular dialects of English (e.g. American vs. British vs. Australian)?
Are there even any differences? My rationalization for encountering both words would be that there must be a subtle difference as explanation why both words exist concurrently/