A first version of the plugin is now available here:
The mapping is still very simple, just the basic fields of users and groups will work. I tested with the keycloak SCIM plugin and you can already add/modify users on Keycloak which will then be crested/updated on Discourse. The keycloak SCIM plugin is here:
Any feedback is very welcome 
We will test more with keycloak in our environment and I still want to test it with authentik.