Xbxo Scarlett will be backwards compatible with all Xbox One games, as well as the current Xbox 360 and OG Xbox backwards compatible games on the Xbox One. Current accessories will also work on the Scarlett. Source.
From what little we know about PS5 and Scarlett both companies are continuing what they have been doing and make them more powerful and even closer to a PC.
But its "new" on the playstation side.
While Xbox One already had selected xbox & xbox 360 titles for it.
With the PS5 its supposedly all titles, and its a first time for such.
It absolutely isn't "new" for Playstation. The short memories of people these days. If anything it's the other way around.
PS3 launched with nearly full BC while the 360 had very limited BC at launch. It wasn't until later in the gen that Sony dropped BC to cut the cost of the consoles. It definitely isn't the "first time for such." I was able to play all my PS1 and PS2 discs on my OG PS3.