Short answer: https://www.youtube.com/watch?v=KuCvH-fyDns

Long answer:

Josuke.

Josuke has been an unexpected gamechanger. You can draw some parallels between Josuke and the other JA JoJo, Giorno Giovanna, with their multi-hitting ultimates that rollback enemy gauge and their primarily support movesets. What sets Josuke apart is that he's FAST (so doesn't rely on a mediocre other unit like Bruno to make him viable) and he has synergy with the most dominant single color in the game (Sharing space with Dai, Vivi, SSJ2 Gohan, SSJ Goku, All Might, etc). This speed enables him to outpace most other common rollback units. IIRC only Tezuka can outrun Josuke, and I believe he needs Mihawk, Tsubasa, AND Yuuma to do so (but I could be wrong on that). That means former dreaded units like Misa, Aya, etc. simply aren't able to get any of their cooldown altering moves off before being either rolled-back themselves or outright killed by Josuke based on how you have him built.

Frankly Josuke allows HRT to counter Cool Head in ways they couldn't before making the matchup a bit one sided.

Cool Head vs. Josuke

• Unable to roll back their cooldown due to Neuro, but will still roll back/stop Misa

• His ultimate ignores Defense allowing him to break passive on (or outright-kill) Kurama and Neuro

• 5-hit ultimate will break all but 1 of Light's shields and will generally kill Sasuke THROUGH his Susanoo shield skill.

• Cool Head teams can no longer tech in Kinniku to counter HRT as Josuke's ult will hit through Kinniku's team-DEF passive

• Any faster threats to Josuke like Killua are easily killed off by Dai before they can make a move.

How to build Josuke

There's a lot of options, but Josuke's ult being able to break Cool Head teams is IMO its most valuable asset. That said, I don't recommend ANY investment via tank koma. Imo the best setup for Josuke is a combination of Crit Rate koma (being able to make it to 50% crit rate with only 2 stacks) and Ult Damgae Koma. The ratio is up to you, but personally I run 4 Crit koma and 2 Ult koma.

The end result is a team that keeps Cool Head from being able to do anything at all

That said he's not a slap-on God Tier in any team build. He needs his support just like anyone else. So for any player invested in HRT teams, he's a must have IMO. For those who don't have a strong HRT build or are lacking in Dai and Vivi, Josuke probably isn't the answer to Cool Head for you.

Also, I don't expect the volume of CH teams to dip all that much. The Ichigo-Yusuke counter team already dismantled them and didn't do much of anything to cut into their sheer overwhelming numbers. CH is simply easier to build compared to these other teams as long as you're been playing for a while which will definitely result in more people continuing to use it and its numbers to remain consistently high. That's a good thing for Josuke though!