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This issue has been migrated from #15645.
MSC2000 was closed, we should likely remove support for it at some point. It will be obsoleted by the OIDC work anyway, so we can likely wait until that removes a bunch of the auth code, but it seemed worth having a separate tracking issue.
Note that this landed before we had processes around handling experimental code (from MSCs) in Synapse so it landed with stable identifiers, etc. See #7118 (originally #5214 in the dinsic branch).