Stealth is in the game. It's almost mandatory if you care about getting high ranks for certain verses. I hope you don't mean that it should be be centerstage like in the original MGS games. MGR is meant to be a character action game not stealth focused.
It was a very lightweight version of stealth included in MGR.
I don’t mean it should take center-stage, as if we are playing MGS2, but I think there is an in-between achievable there.
Stealth actually was a larger part of the original game’s concept before Platinum took over. It didn’t work out but I think there is potential for it.
Both this game and Sekiro - character action games - have implemented very light stealth. It’s just not the kind of stealth that gives you any agency, so it’s not satisfying.
The stealth is basically already there, they’ve made room for it. Now they should make it fun.
Frankly, I don’t believe there is a rule that says a character-action game can’t also feature stealth mechanics. It’s just somewhat rare. Mixing game mechanics together is a way to innovate and create new experiences.