I think it's unlikely PS5 will have much in the way proper BC like Xbox has had this gen.
Possibly PS4 discs will work but other than that I wouldn't expect much from them at all, they're likely to go the same way ninty has with making people pay for old games over and over again with no other option.
By all indications Xbox Series X will have the same BC capability that One has, although I'm not sure if One physical games will install from disc on SX or just have inner ring read to download digital version like Xbox & Xbox 360 games that are BC.
