Aight. Long ass post incoming as to why Clefable should stay in A+.
There are three things I mainly want to cover with this post: Clefable's stats, its roles (ie versatility), and its "splashability."
So lemme start off with clefable's stats, as they are the biggest hindrance to it. I'll be focusing on its 95/73/90 defenses since they are the most important when it comes to what it does. Quite frankly, they're shit. For a defensive mon, Clefable is lacking. Really lacking. It has less overall bulk than Keldeo. Let that sink in. Now let's take this bulk into real game scenarios. Clefable is used to check prominent threats in the tier such as Latios, Keldeo, Thundurus, Raikou, Mega Manectric, and others. It has two main EV spreads — physically defensive (for CM), and mixed defense (for pure support roles). The problem with these is that if Clefable is using the wrong spread, the very things it checks beat it. PhysDef loses to ever special attacker it's supposed to check bar Latios, while the mixed defense sets can be broken through by naturally strong physical attackers. Furthermore, the hits that clef do live are doing around 40-49% on average. Sure, you're avoiding a 2HKO so you're all good right? Nope, not really. Clef has terrible speed, and on top of this, literally any prior damage makes it a liability. Think of it like this. Clef switches into Manectric so it can wall it, except Clefable ends up eating a volt switch. Now, a Clefable check is brought in, forcing clef out. Guess what? Clefable can no longer switch into Manectric because t is not 2HKO ed by thunderbolt on the switch in next time. This is what is holding Clefable back from S. It can't reliable check things throughout the match because its stats are too low to be able to do so consistently. I will admit that magic guard and softboiled definitely help with this and mitigate this issue. However, what if you're using unaware? Now you're fucked by stealth rock and spikes and burns and toxic and everything else. Clef can't check half the shit it's supposed to when it's taking sr damage. On top of this, Clefable cannot simply take on the powerful attackers of the tier (the wallbreakers). Mega Garde, Megagross, Landorus, Specs Keldeo, Bisharp, Diggersby, Medicham, etc all 2HKO it with a neutral hit. For a Pokémon to be S rank under the support category, this is simply unacceptable. Yes Clefable does have teammates. However, having to rely on teammates to check the most powerful threats in the tier is not a good argument for those that want it to raise. I realize that bulk alone does not define a mon, but when you're talking about a defensive Pokémon, it is one of the most important aspects about it. Clefable simply doesn't have it.
Next, I'll delve into the versatility. Clefable has three "main" sets: MG CM, Support (includes SR, cleric, etc), and Unaware wall. That's really it. Sure you have lure sets but they're really not that great. Yeah they lure in shit but this variant of clef dies so fucking fast it's not even funny. Plus it's not even that strong (lol @ base 95 SpAtk). Anyways, MG CM is arguably clefs best set. And it's a damn good one. It's easily A+ worthy and I will not deny that at all. However, it's not S rank worthy. Despite the fact that it is an excellent sweeper, it still has its stops, and those stops stop it in its tracks. It's problem lies in the fact that it it really slow and relies on at least two boosts to deal any meaningful damage. This, coupled with the fact that it only has one coverage slot (which il get to in a minute), means that no matter what, it will be walled by something. It is also shut down by taunt and is revenge killed pretty easily by the abundant steel types in the tier. Now, Clefable does have its ways of getting past certain checks. T wave fucks over faster things that need their speed, but it still leaves clef walled by Gliscor, and it doesn't entirely stop steels from killing you (you still die to them). Flamethrower means any non Heatran steel can't safely switch in, but then you have mons like Gliscor and Talon that best you. Ice beam is cool until you realize that steels and mega Venu still beat you, and TBolt is ass lol. It doesn't even 2HKO SpDef Talon w/o a boost, and roost removes the weakness + taunt is a thing. Also
BKC why would you run SR (or HWish) CM? Like that's literally the worst thing you can do because you're literally inviting every Talonflame, Scizor, Manaphy, Skarmory, etc to come in and set up in your face. I respect you as a player but honestly that is a terrible set and I don't get why you're even bringing it up. Anyways clef relies on its teammates to take on its counters. Normally this is a weak argument, but you have to consider that Clefable has a shiton of checks and counters because of how prepared the meta is for it. Every competent team will have ways of dealing with it because otherwise you're asking to get swept.
The support set can do A crapload of things like cleric, lay sr, t wave, what have you. However, this set in particular is extremely passive and cannot really do much back to its foes aside from maybe para them or spam recover. It is also complete setup bait unless it runs unaware, which leaves it susceptible to hazards and as I said earlier hazards fuck it up completely. I will also touch on unaware here. It's a good set, what with it countering dd alt, tg mana, etc. however, while it does wall setup sweepers it can't wall natural wallbreakers, especially special attackers. Furthermore, hazards and toxic wear this set down, especially when it has to run either wish tect or moonlight for recovery. This either gives it opponents free turns or puts it on a timer. And in longer matches, the latter means that clef is a liability after 6 turns of moonlight (because now x setup sweeper can stay in and force it to use its last two recovery moves then break clef and sweep the team). Let's also not forget that clef can no longer switch into Keldeo or Manaphy with impunity anymore why? Because scald is a thing. Even unaware CM sets aren't perfect (quite frankly this set is extremely overrated and quite shit in practice). If they get burned they're done for because they're put on a timer because of burn and limited recovery. Add on the fact that this variant is
extremely hard pressed for moveslots (if it wants a coverage move, it has 8 PP of recovery. If it wants wish tect, every steel type ever walls it bar Bisharp. If it goes for heal bell, literally anything that resist Moonblast and can status / deal meaningful damage to it can beat it). Support sets are good, but are not nearly as good as the MG CM set and in no way S worthy.
Finally, it's splashability. Is this honestly a reason to move it up? I mean I don't think it is. Despite being a great glue mom that can hold teams together, clefs ability to fit on many balance teams isn't a reason to move it up. It's a glue mon, and a damn good one at that. But that doesn't make it s rank. By nature if being a glue mon, it's forced to hold a team together, which means it can be overloaded at times. This isn't a good thing because then it's incredibly easy to pressure with a competent team and it will be broken eventually.
I get it. Clef is a cut above the rest of the A+ ranks because of how good it is. But it's not S rank worthy. The only way it would be is if there was an S- rank and then it could go in there alongside Keldeo. But that's been struck down repeatedly so I won't bother talking about that anymore. Clef simply isn't meta defining like the rest of the s ranks. It's fairly easy to keep it in line both in the team builder and in battle. It's strength comes from its utility, but that's really not a reason to move it up.
TL;DR keep clef in A+.