As always, it’s impossible for the game to know whether dropping out mid-round is a ragequit or if there’s a legitimate reason; whether the players have discussed about it beforehand (in game chat, through game’s voice chat or through external applications like Discord), or possibly even whether it was a bug or a sudden disconnection.
As such, heavy penalties really shouldn’t apply here, even if it is annoying for the others. A debuff for the host who quit is possible, but might end up still penalizing unintended disconnects, or ones for genuinely good reasons. A blacklist has been a long-asked feature (or at least using Steam’s existing block feature), and could help against repeat encounters at least. A reputation system could be the most effective one, but that kind of stuff is still easily left open for abuse.
Personally, I think rather than penalizing the host who left, a better solution could be compensation for the others who suffered, likely in the form of equal EXP to a failed run, as that’s what it essentially was, and certainly what causes ragequits (well, more accurately the impatient host predicting a failure or getting bored with someone else’s clutch).
Of course, a checkpoint system and good host migration would help with these, and dedicated servers might mitigate at least some issues depending on the implementation, but as you pointed out, we haven’t heard anything in a long time.