The upcoming Xbox Scarlett will be fully compatible with existing controllers and other peripherals used today with Xbox One.
One of the major points Microsoft stressed during the Xbox Scarlett presentation at E3 2019 is backwards compatibility. Just like legacy games will be compatible with Scarlett, so too are accessories in use today.
The platform holder confirmed this detail in a blog post following the console's reveal. It also pointed out that your existing "Xbox career", likely meaning achievements and friends list will carry forward.
Speaking on the Inside Xbox show, head of Xbox Phil Spencer also confirmed that original Xbox, Xbox 360 and Xbox One games will be playable on Scarlett. That obviously doesn't mean every single game will work, as you still have to account for licensing and other business considerations, but it's a big step nonetheless.
Microsoft just rolled out the final backwards compatibility update for Xbox One, bringing even more original Xbox and Xbox 360 games to the fold, including classic Splinter Cell.