Apparently to bring usernames and channel custom names back together, YouTube is introducing unique identifiers called handles which are basically an at symbol (@) followed by either username or channel custom slug name. See their introduction video. Handles will be rolling out soon and will be automatically assigned starting on November 14, 2022 using usernames and channel custom names if not manually chosen before.

The specification can be found at https://support.google.com/youtube/answer/11585688#tns:

• Is between 3-30 characters
• Is made up of alphanumeric characters (A–Z, a–z, 0–9)
  • Your handle can also include: underscores (_), hyphens (-), periods (.)