now don't get me wrong, I'm a huge toxic skarm fan. being able to "limit" zapdos like that is huge, as well as criple aero that you come into. toxic is deadly in faster mus. however, in slow mus your enemy will carry more dedicated status absorbers like nat cure mons. if gengar shows up in those mus, its often going to be the biggest progress maker on the enemy team, so letting it in is a huge liability. you might also run across refdol which blanks tox skarm via pp stall where peck skarm can make progress. and lastly, forre tends to shine on slower teams.
now why does it matter more to have a move to win slow mus than fast mus? it's because in slow mus there's more space to scout. in fast matchups skarm often dies without revealing tox/peck, or reveals these moves super late. as you point out, toxic makes skarm absolutely terrifying to switch into, but the thing is, if you're not scouted, you don't actually have to run toxic to inspire that fear. you can just bluff tox and your opp will be forced to do stuff like go gar instead of zap, even if you don't actually have tox. in slow mus, after they've scouted you (which they can do more easily since games are longer), they can exploit your lack of peck harder than they can exploit your lack of toxic.
if you watch a lot of skarm gods play, they like to hide their 3rd/4th move until late to maintain an information advantage. and tox skarm benefits less than peck from this style of play.
that said I like tox skarm a lot on certain teams, like if i have a gar or a suittar, or if im softer into fires/electrics, I'd probably favor tox. and protox lets you handle fat mie more conclusively. so it's not clear cut by any means.
