Keldeo has been getting so much traction lately, and I believe it only got better with the Pheromosa ban. It is imo the best Scarfer in the tier, and it can hit like a truck with its Specs set, or even serve as a win con with its CM + Normalium Z set. In nearly every team I've built recently I've found myself throwing on Keldeo because it covers so much and I barely lose anything from doing so. It has great synergy with a lot of popular mons like Volcarona, Mega Mawile, and Greninja.
General Strengths
Keldeo is probably the best Scarfer in the tier. It outspeeds every other relevant scarfer bar Gengar (though that has taken a hit in viability with Pheromosa leaving) and Greninja (which is an uncommon set that Keldeo beats if it isn't running Extrasensory). It speed ties with Terrakion, but that isn't common and you live a hit from anyway and can OHKO it in return. It's super splashable, and can be customed to your team. Weak to Volcarona? Add Stone Edge on Keldeo. Weak to Gyarados? Throw on HP Electric. Salamence? Icy Wind! Icy Wind also allows it to slow down threats in a last resort situation. It can spread burns with Scald, threaten Special walls with Secret Sword, and throw off more powerful Water hits with Surf/Hydro Pump. It's not the easiest thing to switch into, with its main switch in being Tapu Fini, which is super easy to wear down. Water/Fighting coverage is great in general, and the OU mons it can't hit like Gyarados die to HP Electric, or can be dealt with by its teammates. Aside from its Scarf set, its Specs set hits like a truck. It has a good Speed tier, and absolutely nukes mons, such as:
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 4 Def Celesteela: 202-238 (50.7 - 59.7%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 4 Def Tangrowth: 171-202 (42.3 - 50%) -- 92.6% chance to 2HKO after Stealth Rock
252 SpA Choice Specs Keldeo Hidden Power Electric vs. 252 HP / 24 SpD Toxapex: 142-168 (46.7 - 55.2%) -- 99.6% chance to 2HKO after Stealth Rock and Black Sludge recovery
252 SpA Choice Specs Keldeo Hydro Pump vs. 248 HP / 0 SpD Zapdos: 292-345 (76.2 - 90%) -- guaranteed OHKO after Stealth Rock
252 SpA Choice Specs Keldeo Hydro Pump vs. 240 HP / 212+ SpD Zygarde: 207-244 (49.6 - 58.5%) -- 99.6% chance to 2HKO after Stealth Rock and Leftovers recovery
252 SpA Choice Specs Keldeo Icy Wind vs. 240 HP / 212+ SpD Zygarde: 276-328 (66.1 - 78.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Calm Mind + Normalium Z can catch people off guard, and Keldeo can blow past its common answers of Tapu Fini and Toxapex with this set.
Keldeo also has pretty good bulk for an offensive mon at 91/90/90, and allow it to live hits/take less damage you wouldn't expect. For example:
252 Atk Aerilate Pinsir-Mega Quick Attack vs. 0 HP / 4 Def Keldeo: 194-230 (60 - 71.2%) -- guaranteed 2HKO
All in all, Keldeo is a splashable, devastating revenge killer and late game cleaner, with its common answers able to be worn down relatively easily, and should move up to A+.
SPL
It got 13th place in usage in SPL, and has a ~60% win rate. Of the top 20 most used mons, it has the third highest win rate. In one of the most prestigious tournaments with highly skilled players, this Gen 6 tyrant has managed to still make a splash. It has a higher win rate than other A+ mons like Zygarde and Celesteela, even. People thought this mon was a lot worse at the start of this gen, and it didn't see much use. However, once its Scarf set started to pick up, it picked up a lot, and Keldeo became super common in tournament. Whenever usage stats come out, Keldeo will probably rise back to OU, and if not this update, the next.
Pheromosa
Finally, one of the most restrictive tyrants of OU has finally left. Pheromosa leaving has caused Greninja to start running wild. Keldeo is arguably the best offensive check to Greninja, since it can live any hit from either Greninja bar Extrasensory and OHKO with Secret Sword. Pheromosa leaving also makes Toxapex less common, which is one of Keldeo's best answers. Scarf Gengar has taken a hit in viability, and that was one of the only scarfers that could outspeed Scarf Keldeo.