I think "dependence" and "dependency" are like "competence" and "competency". Both are nouns. Yet, "dependence" emphasizes the quality of being dependent. So, it may also be abstract. Whereas, "dependency" focuses on the state of being dependent. It is likely to be concrete.
This seemed reasonable at first, but I'm not convinced it's the best explanation.
The conclusion at https://oneminuteenglish.org/en/dependence-or-dependency/ was a bit clearer, although mostly the same:
Just remember that “dependence” is the quality and “dependency” is the state of having to rely on someone or something else.
https://ell.stackexchange.com/questions/41528/differences-between-dependence-and-dependency/41561#41561 disagrees with the claim that "state" makes it likely to be concrete. And I like how it clarifies state as "state/condition" (condition is a lot clearer to me):
Dependence and dependency can both be used in the state/condition of being dependent sense. By definition, all words referencing such "states" are abstract nouns, so I don't see any justification for OP's abstract/concrete distinction in that sense.