i think a really attractive part of banning conversion is that nothing aside from porygon learns it. you can consider a porygon ban and a conversion ban synonymous in terms of nerfing por-z-gon, with the former banning other porygon sets. i think with the developments in the meta so far and basing off past LC metagames it's fair to assume porygon is a good presence in the meta; this should ultimately be a minor consideration but i don't think it shouldn't affect policy at all, considering that smogon is more open to considering what's "healthy" for the metagame in its bans nowadays).
The main thing about this is that it goes against the precedent set by higher tiers and smogon as a whole. We can of course break from this, although it might get some flack, but if we look back it's been pretty consistent that if it's a an element on a pokemon that's arguably making it broken then it's the mon as a whole that goes. For example King's Shield on Aegislash was thrown around a ton but shot down pretty hard.
The main reasoning for this is the whole idea of not banning sets/nerfing mons to keep them legal. This is the same thing as why Blaziken is banned and not Speed Boost Blaziken, why Greninja was banned and not Protean Greninja (or Protean in general!), and in LC why Yanma was banned when a majority of the council reasoning was that Hypnosis+Compoundeyes was the biggest issue rather than the Speed Boost sets.
So, for Conversion specifically, the issue here is that the move itself isn't broken. Conversion when used without the Z Crystal is actually pretty bad, and far from broken. The issue is that Z-Conversion on Porygon gets pretty busted, but unfortunately we can't ban Z-Conversion mainly because it'd have to be an in-battle modification. This actually isn't without precedence, so it's debatably an option - the precedence is actually Mega Rayquaza, where the ban was clicking the mega evolution button on sets with Dragon Ascent rather than banning the move itself. Of course that was banning a form instead of a move, so it's not a perfect comparison, but the (loose) precedent is there.
However, is Z-Conversion broken on Porygon or is it a broken move? While no other pokemon gets conversion, we "luckily" have moves that function almost identically on mons in LC. For example Z-Trick or Treat does the same thing in a +1 to all stats, without the conversion effect. Unless people are arguing that conversion's type-changing effect is the biggest reason that Porygon's busted with Z-Conversion then we have equivalent moves. If people can agree that the other +1 to all stat moves are broken then there's a much stronger case that Z-Conversion is a viable banning target (along with all other moves like it) but so far that hasn't seemed to be the case. +1 Pumpkaboo is certainly an interesting mon, but not a particularly fantastic one.
So why is Porygon with +1 Z move so much better? In the end, it really seems to come down to the fact that Porygon has great bulk, fantastic special attack, just enough speed, a very good special movepool, and Z-Conversion. While you obviously can make the case that it's Z-Conversion breaking the mon you could do the same for any of the other characteristics I mentioned, making a conversion ban a clear nerf to Porygon. After all, maybe it's actually Download that's the issue - Porygon's the only one with Download so it's not like there'd be any collateral damage for banning that either.
Maybe I've missed something in playing with Z moves that give +1 to every stat and they're actually broken in general. If you can argue that successfully that'd be a good way to maybe go for a +1 Z-move ban. However banning the move conversion would be a direct attempt to nerf Porygon by banning a move that by itself isn't broken - you might as well argue that Download is the real issue and should be banned. I'd hate to see Porygon go (it's a fun mon and without it freemissy becomes harder) but that's the most logical option in my mind if it's indeed deemed broken.
@Cutiefly/BP unless chains lacking Cutiefly are seen as problematic for a lot of the same reasons above Cutiefly should go. We shouldn't ban a move just to save one mon, no matter how cute it is. I think there's actual potential to be argued for BP in general being dumb, but don't ban BP to save Cutiefly. Complex bans or clauses, on the other hand, suck.