Unpopular opinion of mine:
Fairy type is not broken. Rather, we got too many *good* Fairy types in last 2 gens.
I went a bit out of curiosity to check the success of Fairy types in this year VGC.
According to VGCstats, those are the fairy types who obtained the presence in top 25 CP winning teams.
Sun series:
- Xerneas at 2 with 53%
- Tapu Koko at 7 with 26%
- Tapu Lele at 8 with 23%
- Tapu Fini at 13 with 14%
The next closest is Clefairy at 28 with just barely off 3%
Moon series:
- Xerneas at 2 with 47%
- Tapu Koko at 3 with 39%
- Tapu Lele at 12 with 18%
- Tapu Fini at 19 with 9%
- Whimsicott at 21 with 7%
The next closest was Bulu at 36 with basically just 1 round
Aside from the omnipresent Xerneas, Tapus and couple niche options (Whimsi bringing Prankster Tailwind / Z-Nature Power, Clefairy's Follow Me, etc), we don't actually really see other Fairy types at all. Other formerly popular fairy types like Alolan Ninetales dropped in usage very fast.
Even when you look at Smogon tiers, the only Fairy types viable in OU are (Mega)Diancie, Clefable, Magearna, Mega-Mawile and the Tapus, with the odd appearance of A-Ninetales, Ribombee.
They either are blessed by having some overpowered feature (Geomancy, Huge Power, terrain, Steel/Fairy typing, ..) or particularly strong niche (Veil, Webs)
And Ubers actually don't feature a single fairy type aside Arceus, Xerneas and some of the OU ones.
Other fairy types slowly but surely plummeted in bottom tiers, and if they are relevant in a tier it's generally due to again a specific niche/ability rather than just because they're a fairy type with BST to back it, expecially as poison types are more common in lower tiers than they are in OU/Ubers or VGC.
While Fairy typing is definitely a strong type, I think the issue is more that they made very strong fairy types in the last 2 gens.
They actually overcommitted in making strong answers to Dragon types, making combinations of very strong Pokemon with an also strong type to back it out.
I don't think that *Fairy type itself* really needs a nerf, I think what the game really lack is an answer to those strong Fairy types. Steel/Poison coverage right now are often suboptimal (as they really lack useful supereffective or neutral coverage compared to just running stabs), and strong Poison types are particularly rare to begin with, with Fire types who would otherwise resist Fairy being cursed by the Stealth Rock weakness.