Keldeo really hasn't gotten any new toys since its debut in Gen 5, and unfortunately Gen 8 doesn't really break that trend. Keldeo received no notable new moves and doesn't benefit all that much from Dynamax, outside of being able to set up rain. Toxapex is still just as widely used as ever, and in official formats teams often pack multiple Water resists and immunities to deal with powerhouses like Dracovish, which has the side effect of making teams very hostile towards Keldeo. That being said, Keldeo can still put in work as either a Choice Specs user or a special sweeper who can't be stopped by Chansey/Blissey, but needs a significant amount of team support to actually pull of sweeps against many teams, and is rather match-up dependent.
Keldeo @ Choice Specs
Timid - Justified
252 Speed, 252 Sp. Atk, 4 Def
-Hydro Pump
-Secret Sword
-Icy Wind
-Scald
There's not a whole lot to talk about with this set, really. Keldeo's movepool is very barren, and these are about the only 4 remotely viable moves he can run on a Specs set. Hydro Pump is his strongest STAB, and does admittedly put big dents even in things that resist it. Secret Sword is a move exclusive to Keldeo that esentially acts as a Fighting-typed Psyshock, calculating damage using the user's Sp. Atk and the opponent's physical Defense, letting him deal serious damage to dedicated special walls. Unlike the majority of other Water-types, he doesn't get Ice Beam and must settle for the significantly weaker Icy Wind. Finally, Scald offers an accurate alternative to Hydro Pump, and really there isn't a whole lot else to fit in that moveslot.
A big problem for Keldeo is the popularity of other Water-types like Barraskewda, Gyarados, and especially Dracovish. Answers to these Pokemon in the formats where they can be found are likely to be on every team, and it just so happens that most Pokemon who can handle those threats can also handle Keldeo pretty well as well.
Keldeo @ Leftovers
Timid - Justified
252 Speed, 252 Sp. Atk, 4 Def
-Scald
-Secret Sword
-Substitute
-Calm Mind
This ius another set that has existed since Gen 5 and its use cases haven't really changed very much since then. Come in against something that Keldeo can use as set-up bait, such as a Pokemon it scares out or a bulky Water-type not named Toxapex, set up a Substitute, set up some Calm Minds, and then try to pull of a sweep. Its coverage with its STABs alone isn't bad but does miss out on some key opponents, such as the aforementioned Toxapex as well as Dragapult, Dragonite, Pelipper, and a few lesser used Pokemon like Decidueye and Jellicent. Icy Wind could be carried to hit some of these threats (mostly the Dragons and Grass/Ghost types) but it means giving up Substitute, which robs him of many set-up opportunities, and Icy Wind fails to KO any of these opponents without at least one boost, sometimes several. In addition, nothing in Keldeo's entire arsenal will let it effectively bypass the more defensive answers, like Toxapex or Jellicent.
That being said, Keldeo does make a decent clean-up sweeper when these opponents are weakened or removed. Secret Sword targetting the opponent's physical Defense turns Keldeo into a pseudo-mixed sweeper, letting it bypass Pokemon who are usually hard stops to special sweepers, such as Chansey and Blissey.