despite the fact that its hitbox was slightly decreased. It still works basically the same.
it's not that simple, though. it had a disjointed hitbox that extended beyond her hurtbox before and now it doesn't, that's a big deal. that means when you trade hits with someone, you always get hit, whereas before you could outspace your opponent's attacks with her nair and that is entirely impossible now. also, shff nair covered rolls toward you, but now that is also impossible due to the smaller (or lack thereof) hitbox on her butt.
this might be an exaggeration (for some, not for me) but, imagine if you landed 10 nairs in a match. lets ignore move deterioration, and just add up how much damage is lost. that's 25% lost, and if you consider most characters are in kill zone around 100% that's increasing a quarter of a stock's life, and in a 2 stock match that's a significant amount. i can't say which ones for certain but i know there's some projectiles rosalina can no longer negate with nair now, too, due to the lower damage. this is on top of rosalina's overall kill potential being nerfed all around, usmash more notable but fsmash as well.
the knockback is now worse than luma's initial little poke in the beginning of the nair. it's hilarious. you can kill like 30% sooner just with luma's little tiny poke. also, this doesn't just affect just it's killing power but also it's ability to get the opponent off the edge, which obviously is a big deal. how many kills do you get with rosalina off the edge? i get at least 50% of mine from dair/whatever once a person is knocked off. this scenario is set up far less often. i can't come up with any numbers except 'a lot %'.
rosalina's neutral game now is so meh. what options do you have other than to just shoot luma out? luma is like an annoying bug that gets out prioritized like it's nobody's business, and if you send luma out you're risking losing your most valuable defensive and offensive asset. are you going to dash attack? that move, if shielded, is incredibly punishable. star bits? again, laggy and easily punished on shield. rosalina's one move that is almost safe on shield is dtilt, and now it can be rolled into and punished.
now i think in high level play rosalina's #1 neutral game is defense. this sucks in a lot of situations. nair has two uses i'd say, and that's retreating sh nair (in some MUs) and covering jump/stand up on ledge getup. as an approach it's just not worth using. it's unreliable to sh nair in place. shff nair is garbs in general. rosalina has no combo breaker move, either, she never did, but i felt nair's strength overrode the need for one of those 'get off me' attacks.
something else, i'm exploring other options. autocanceled luma stuff might be the answer for an aggressive rosa but i can't say. sh dair is purdy good for getting easy autocancels. but i dunno.