Is foliage with 6k tris per Unit a thing to do now for aaa games? If LODs are there? Like from Megascans
Probably not.
The engine can’t even do 144hz at 4k on a good day with an empty scene and a 5090.
the thing is, if you are making the models you can reduce that tris count drastically and still have something entierly workable (though maybe in a different engine? 
).
As always anyway, it depends on quantity.
1 instance popping in with 6k tria? That’s literally nothing.
1000 instances of 6k tris? That will drag the render down or possibly stop rendering (there’s a max to the engine displayed tris, so when that exceeds it stops loading stuff).