Action games and shmups.
Luckily, people who played classic action games on the PS2 growing up are old enough to start making their own games now, so we're getting a bunch of indies like Genokids and Spirit X Strike, but except maybe the once-in-a-decade KOEI TECMO or Capcom release, big studios turned their back on the genre over profitability concerns, I assume, even though Devil May Cry 5 sold over 10 million copies.
Shmups: you know you're fucked when even the most legendary shmup developer—CAVE—moves on to make gacha mobile games. Other than the occasional Touhou release, it's all in the hands of indie developers now. Going from a time when companies as big as Capcom were making shmups to now is difficult to stomach.
I'm tempted to throw fighting games in there, but the genre is bigger than ever, TBF. I'm just bitter it's moving in a direction I don't like, which's a personal gripe more than anything.