To be pedantic (the best kind of antic) it should only back deploy when your minimum deployment target requires it. So to expand the phrase: “the function is back deployed <when your minimum deployment target is set to> before iOS 13”. There is not a similarly smooth expansion in the "up to" case (let me try... "the function is back deployed <when your minimum deployment target is set to a value> up to iOS 13"... not great).
Note that this kind of expansion is pretty normal e.g. "Up to" is really short for "up to but not including" but that meaning is supposed to be given as a term of art (though just now, someone in our office chat just said "IMO it isn't immediately clear whether upTo is inclusive or not" :) We should not try to stuff more words into it, either way, just try and find the best short word (or two if we must) that sounds right and most people get the right inference from.