Just relying on optimistic check of fork choice head is not sufficient at least in case of Lodestar as last imported block is flagged as valid if EL was synced when importing it and the beacon node will only consider itself as syncing after head is sufficiently far behind (32 slots).
It might be best to report a 206 immediately to allow health-based fallbacks to be triggered asap.
Follow up to https://github.com/ethereum/beacon-APIs/pull/442 to clarify how to handle the case if EL is offline as beacon node will not be able to serve all requests in that case.
Just relying on optimistic check of fork choice head is not sufficient at least in case of Lodestar as last imported block is flagged as valid if EL was synced when importing it and the beacon node will only consider itself as syncing after head is sufficiently far behind (32 slots).
It might be best to report a 206 immediately to allow health-based fallbacks to be triggered asap.