[Clarification] Skipped slots in /eth/v1/beacon/blocks/{block_id} endpoint

Should skipped slots return a 404 in the `/eth/v1/beacon/blocks/{block_id}` endpoint? In Lighthouse we return information about the most recent non-skipped slot presently. I don't see it explicitly in the spec.

Relevant lighthouse issue: https://github.com/sigp/lighthouse/issues/2186

Start by reviewing the `/eth/v1/beacon/blocks/{block_id}` endpoint specification and the linked Lighthouse issue 2186. Compare the documented behavior with Lighthouse's handling of skipped slots, then determine whether the specification should require a 404 or allow the most recent non-skipped slot; done means the endpoint behavior is explicitly resolved in the specification.

Written by the indexing model from the issue text.