[seabass-labrax]

Yet, among those with whom I associate, "Who didja..." is more common than the semantically equivalent elision "Who'd you", probably because it is ambiguous whether the latter is in the past or present tense.

(For context, I live in south-west England, have an RP accent, and 'whom' was genuinely the word that felt most natural to me when writing this post.)

[lupire]

People speak of the future conditional so rarely, it doesn't affect linguistic evolution. "Whodja" being shorter and less stuttery than "whodidja" is a much more powerful driver. Like "Wensday", unless you actually need to disambiguate past from future.