Since TRC already went over the options on the slate and I have no objections to it, I'll just quickly go over all the other abilities, and explain why they did not make the slate.
also a PSA from the TL, please remember that discussion steps are for discussion. as redundant as that sounds, there was a lot of bandwagoning for Water Veil, and while it's not bad to support a option, please remember to at least discuss other options in the future.
Flare Boost - Extremely distracting, doesn't actually help us win against burns on the DD set, and doesn't particularly help the CM set due to it not really addressing the issue of "being burned makes it way easier to kill."
Guts - Just as distracting, only good for the Dragon Dance set, and franky too good of an ability, seeing as how CAP20 can just slap a Flame Orb on and enjoy both immunity to Thunder Wave as well as nearly double the power of a Life Orb.
Hydration - Too situational and rain's pretty meh to be reliant upon for a set-up sweeper without something like swift swim.
Natural Cure - forces CAP20 out, and as a sweeper, you don't want to be forced out since you tend to commit and rarely get more than one or two chances to sweep.
Shed Skin - you don't want to rely on a 1 in 3 RNG chance as a sweeper. as mentioned in the thread, severe reliability issues.
Suction Cups - roar isn't really that common, Dragon Dance really doesn't care, situational, we have other issues with phazers/already beat them.
Liquid Ooze - this is just bad.
Pressure - this beats mega bro and that's about it. Way too narrow.
Marvel Scale - Still cripples the Dragon Dance set, arguably way too good for the Calm Mind set, +50% defense is no joke.
Clear Body - too situational, nobody really cared.
Filter / Tinted Lens / Prankster/ Magic Bounce / Magic Guard / Sheer Force - all of these are too powerful and do not perform any roles that justify their inclusion on CAP20.
Unaware - while this falls into the above camp, I'd just like to isolate this one to bring up the fact that Quagsire, an otherwise extremely lackluster pokemon, actually saw significant use both last gen and some niche use this gen simply because it has Unaware. and CAP20 is almost certainly way better than quagsire in the stat and movepool department.
Mold Breaker - nothing good in OU even has water immunity.
Lightningrod / Levitate / Sap Sipper - these just blatantly ignore the checks / counters list.
Sticky Hold - trick is not a thing anymore, kind of cool against knock off i guess but knock off will chunk CAP20 anyways so double edged sword, too niche.
Quick Feet - this doesn't help at all, for burns calm mind set wont be running speed investment and dragon dance gets crippled regardless, and 25% chance to paralyze is still killer for paralysis.
Synchronize - if we're burned we lose anyways, kind of a nice "haha screw you" but that's not really worth the ability slot.
With that all said, and CAP20 shaping up to be a pretty Pristine War Machine, let's head onto the polls.