Work has started on a community solution for SSO using OpenID Connect over at GitHub - christiaangoossens/hass-oidc-auth: OpenID Connect authentication provider for Home Assistant. You can install the integration using HACS.
Currently, we are in the proof of concept/pre-alpha stage. It’s not ready for your main Home Assistant setup, but feel free to follow the repository as it matures.
Please also join the community discussion on Github: christiaangoossens/hass-oidc-auth · Discussions · GitHub, to share your feature requests and opinions on polls.
Note that this open letter is still important and unsolved, as an official integration would be preferred and the community integration will have a worse user experience (as we cannot change all screens to fit our needs). So, continue to upvote it.