membershipCapabilities

Snapshot this group's membership capabilities: the group context's extension types plus, per member inbox and installation, the extension types each advertises.

These are generic facts you filter to a specific question. For the proposal (app-data-dictionary) migration: use proposalsEnabled to ask "is this group migrated" — this snapshot answers the other question: an inbox blocks migration when one of its installations' org.xmtp.android.library.libxmtp.InstallationCapabilities.supportedExtensions does not contain org.xmtp.android.library.libxmtp.MlsExtensionType.AppDataDictionary. Pair with UnstableGroup.enableProposals.