OfBorg is expected to notify maintainers when a pull request is made against the maintained package (https://github.com/NixOS/nixpkgs/blob/master/maintainers/README.md?plain=1#L59). Providing a GitHub handle is optional as long as an e-mail address or Matrix handle is given (https://github.com/NixOS/nixpkgs/blob/master/maintainers/maintainer-list.nix#L7). So it can be expected that not only GitHub users are welcome to contribute.

However, there is information that a GitHub ID is actually required (NixOS/nixpkgs#254299 (comment)).

In times where developers decide to leave GitHub for good reasons, it seems anachronistic to enforce a stronger vendor lock-in in favour of GitHub than what the maintainer-list.nix suggests. Plus, notifications through GitHub seem to be limited (#636 (comment)), while e-mail and Matrix might be more flexible here. So it seems reasonable to make sure there are no unnecessary hindrances for developers that wish to rely less on GitHub.