For gamers, backwards compatibility is always a "green" flag since this will allow them to play games from older consoles. It has been a cornerstone of console gaming for many years, to say the least.
You can use the PlayStation 4 DualShock controller on a PS5 — but only when playing PS4 games. However, you can use the PS4 controller with any PS5 game while doing remote play on your phone or tablet ...