It seems to me that in terms of regulations this is still compulsory. It is an extra safety feature.
My daughter is now facing the road and it's true that when the rear hook is put on the seat is really fixed.
When she was facing the road I didn't use the hook (which is placed at the level of the feet) in that direction and it's true that the isofix alone wasn't stable enough.

I didn't install the belt for my little one, who still has her back to the road, I think that with the belt of the third attachment point (so the one that comes with the seat and that is fixed in the trunk) it is sufficient. The seat is very good, very intuitive to use and its evolution to successive groups!

I don't know what the regulations say, but in any case, the seat is not perfectly fixed without the belt. If it is not tight, it can move and shake. Basically, the belt provides better stabilisation and therefore more safety.
Aesthetically, it's not necessarily beautiful but it's still one of the best seats.