Your account has almost certainly been assigned a new username already. From the same link:

> Starting March 4, 2024, Discord will begin assigning new usernames to users who have not chosen one themselves. If your username still has a discriminator (username*#0000*), Discord will begin assigning you a new, unique username as soon as March 4, 2024. We will try to assign you a unique username that is similar to your current username.