mIRC could store the full quit message in memory but only show and allow up to the cut off point when connected.
As I already said, mIRC has no way of knowing what the cut-off point will be, so how could it possibly do that?
Using TOPICLEN isn't reliable as it's almost certainly not derived from the same value as "QUITLEN" in any IRCd. Furthermore, on some servers TOPICLEN is not the length limit for quit messages and on others the length limit includes whatever quit prefix the server uses (e.g. "Quit: "), which is something else mIRC can't know.