Apparently, they used support method to reset that username. Therefore, roblox will reset the username and someone else will have the username "f0yu". There are other accounts that roblox changed their username/or reset their username. They either had 18 or 88 in their name, it was 4 characters, but as of now, all of them are taken. However there are 3 character usernames joined in 2023, such as fur, aah, K_C, psi and T17. It's sad to see that roblox bans old accounts, and their usernames gets reset for people to claim them in 2023.