This is a direct quote from the blue post listing the Midnight Alpha patch notes:
Ion can rationalize it however he wants, but the patch notes themselves say the intent is to “better preserve the value and identity of item art from difficult content.”
Unless we’re going to be given some other way for solo players to acquire Mythic appearances (which seems unlikely because that would involve giving us access to gear above Hero track) it seems pretty clear the stated intent is to deliberately cut delvers out of accessing Mythic tints.