The new authentication server (MAS) is at https://github.com/element-hq/matrix-authentication-service (AGPLv3) and entirely self-hostable - e.g. https://github.com/element-hq/ess-helm for the brand new official helm charts from Element, or https://github.com/element-hq/element-docker-demo for a very quick and dirty docker-compose setup i threw together.

MAS provides backwards compatibility for the old Matrix auth APIs for existing Matrix clients, so they do not need to be modified to keep working. However, to get the most out of all the new auth features (2FA, MFA, QR login etc. etc.) then clients will need to be upgraded to speak OIDC natively. Element X for instance is already OIDC-native.

https://areweoidcyet.com has more details.