This framework for this is now in place. Core Facebook and Twitter authenticators have been migrated, and other authenticators will follow over the coming weeks. Documentation can be found here: Adding a new 'managed' authentication method to Discourse
There is no plan to prevent use of the old system, but it is highly recommended that plugins do migrated, so that complex account association logic can be shared.
I’ll use this topic to track the migrations of core/official-plugin authenticators:
Core:
Facebook
Twitter
Github
Google
Instagram
Yahoo
Official Plugins:
OpenID Connect
OAuth 2
Discord (moved to core)
Patreon
LinkedIn
[ ] Ubuntu (deprecated)
SAML
[ ] Azure-AD (deprecated)
Office365 Microsoft Auth
Atlassian Crowd
Steam
[ ] Dwolla (deprecated)