AltAuth has not been updated to recent Minecraft versions. Since Mojang accounts have become completly unusable I have no need anymore for the modification, although the banned and multiplayer disabled accounts functionality should still work.

AltAuth

AltAuth is a solution to allow players with ~~Mojang account~~, (untested) banned accounts and (untested) multiplayer disabled accounts to join online mode Minecraft servers depending on client and server support.
This works by redirecting the session server requests to a proxy specified by the server.

Trusting the AltAuth proxy session server used by the server is required, as a malicious proxy server can use your Minecraft account for up to 48 (Mojang account) or 24 (Microsoft account) hours on ALL servers. You can select in-game which AltAuth proxy servers you trust. A server using AltAuth has to trust its selected AltAuth proxy server that it prevents unauthenticated users from joining. More information on setting up an AltAuth proxy and a technical overview can be found on the GitHub page https://github.com/lixfel/AltAuth.