Here are the main meta developments I can think of that are relevant to which customs should be used:
-Advancing Air Slash is seriously just better than default; Shulk's default Air Slash is a really limited, gimpable recovery which is simply not a problem Advancing Air Slash has, and Advancing Air Slash as an additional benefit gives Shulk a pretty flush ledge game. As said before, Dash Vision has shown serious promise and has legitimate match-up utility over Power Vision; Shulks should be CPing their down special based on MU which was not known before. Locally we have a pretty strong Shulk who has shown me through play that both default and decisive artes are really good for different reasons (and which you take hugely shifts Shulk's game flow; Sakurai and team did a ridiculously good job on that choice IMO) but that hypers, while still good, seem to be the worst option. If I were writing Shulk's moves today, I'd lobby for 1122, 1123, 2122, and 2123 to be included as his four most critical sets, but hypers would be my next pick for important inclusions so probably 3122 and 3123 as well (probably followed by 1113, 2113, 3113, and 1133 as sets no one will ever use but are the most viable of remaining options).
-As .san said, Paralyzing Counter overall has shown itself to be somewhat less worthwhile for Ike than default (default does stuff off-stage or in weird aerial situations), and default Eruption is somewhat less bad than it seemed at first compared to the other options. No one in my region uses Ike so I can't say I'm a huge personal expert on custom Ike, but I know his sets need work.
-Every Mario player I've ever spoken to agrees that not having 1313, 1333, 2313, and 2333 as sets was insane in the first place and is still insane now.
-Pac-Mans I think have through practice about concluded that fruit variants really aren't worth using while default trampoline is at least sometimes (it's a fair trade-off with meteor as it turns out). To answer
Nu~
directly, I would use Distant Power Pellet because I find default Power Pellet to be a pretty bad move for everything but recovery (comboing into it from bell stun is interesting and perhaps the only offensive situation in which the move is non-suicide, but when I play Pac, my personal style is pretty bell-lite though I've seen others use the bell to great effect) and honestly only find it useful at that when I'm in situations where my up-B won't go far enough in one bounce. I'd really prefer to have the huge distance so I can just draw a line really high up off-stage and do a high recovery in more situations than anything default offers me. I don't think default should be a non-option or anything that extreme, but I'd really like to have the option of distant especially paired with Meteor Trampoline.
-The Pits noticed that Striking Flight is kinda good, realistically usable if nothing else.
-As we were discussing above, the Links have concluded (though patches helped solidify it) that the seemingly useless Quickfire Bow is actually his best option.
-While not limited for slots, I think most customs on gameplay has shown that Piercing Aura Sphere on Lucario is pretty awesome, pretty safely his best option on neutral special. This was not how it seemed months ago.
Those are just off the top of my head. I'm sure there's more that could be said.
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To answer the question about Rosalina, as a main of the character who is also pretty deep into this whole custom thing, I feel pretty uniquely qualified to answer that. Customs hurt Rosalina's average match-up but make her more viable overall because they help her in her otherwise bad match-ups. Those anti-Luma moves are mostly (entirely?) had by characters who Rosalina kinda walks all over with customs off, and of course, there's always the direct things like "Ganondorf being able to recover makes him a credible threat in general which of course helps him against Rosalina just like it helps him against everyone else". When you add it up, it's a lot of characters who improve against her for having access to customs, and that really dents her average performance.
Shooting Star Bit and Luma Warp are general purpose good moves, but the main MUs they really do a lot of work is against opposing camp. Default Rosalina has pretty much zero viable options to force an opposing zoning or run-away character to approach her; I'm convinced Olimar is a super nasty secret counter, and in general anyone whose strategy is to just run away from Rosalina and time her out (who has the moveset to support doing that) pretty much sucks to fight with her other than Duck Hunt who without Zig-Zag Shot has zero real options to deal with the "hit down-B" Rosalina strategy (Zig-Zag Shot addresses that REALLY well though). For dealing with more general zoning, Rosalina has low mobility, and if she has to block on her way in, it gets Luma knocked out of position or killed which means that even if she can punish OoS her punish is going to be weak and won't kill before 160%. Her customs give her the additional option of quickly deploying Luma (fast enough that someone far enough away to be zoning you out might get hit!) and of just shooting a pesky projectile that, while pretty poor at zoning itself, is pretty good at making people either approach or, if they're dedicated to camp, deal with it just enough to give Rosalina enough time to corner them. That way instead of playing a frustrating game of pin the tail on the donkey or dance through the minefield with a huge, slow character who gets punished extra for blocking (and super extra for spotdodging which is guaranteed not to cover Luma!) I can create footsies situations where my character actually performs well. Some people like to imagine that Grav Pull by itself means she wins against all opposing projectiles, but of course, smart players who main projectile characters will usually respond to her grav pulling one by just firing another which if your projectile is actually good is probably an action that will be frame advantageous to you and not to Rosalina.
Customs just lead to Rosalina being involved in more quality gameplay all around; even if I didn't care about her tier position at all, her meta is just a lot more pleasant for everyone involved when customs are on. I don't think customs on tournaments have shown Rosalina being used particularly more than customs off (she's not really that popular of a pick despite how much hype she has as an allegedly super overpowered character). I know I've never been able to create any abusive, easy win situations with Shooting Star Bit or Luma Warp in all my time playing custom Rosalina; as long as your opponent is on the level to have a basic concept of how to deal with any projectile in general and as long as your opponent knows anything about spacing at all it's really hard to milk those two moves very far since honestly I can really only use them more than rarely if it's as a response to my opponent's tactics which isn't something I have full control over.
