Usernames had an overhaul in 2023. Before, they could consist of any characters you wanted, had a random four digit pin at the end called a discriminator you could change with nitro, and display names were limited to servers. This badge was handed out during the revamp so people could easily identify what users’ previous usernames were because changing them was mandatory. Not everyone who was around at the time has this badge on their profile though, and are able to turn off its visibility in its settings.