100% Backwards Compatibility has already been confirmed for both consoles.
Mark Cerny said PS5 will be fully Backwards Compatible with PS4 and PSVR in the Wired Interview Earlier this year.
Phil Spencer announced that they were pulling the B/C off of XBO/XBO X at E3 to assure that all currenlty available XB/360/XBO games would be available on Scarlett from day one.
There is no question of if they will be fully B/C, but how improved will the expereince be on B/C titles.
Stop hate, let others live the life they were given. Everyone has their problems, and no one should have to feel ashamed for the way they were born. Be proud of who you are, encourage others to be proud of themselves. Learn, research, absorb everything around you. Nothing is meaningless, a purpose is placed on everything no matter how you perceive it. Discover how to love, and share that love with everything that you encounter. Help make existence a beautiful thing.
Kevyn B Grams
KBG29 on PSN&XBL