https://github.com/ipfs/specs/blame/937754aa4b7aa96763be40e1503a4fda3f766ad6/src/http-gateways/path-gateway.md#L401-L402

No matter if TTL value is known or not, implementations should always send a Last-Modified header with the timestamp of the record resolution.

and the link points to https://specs.ipfs.tech/http-gateways/path-gateway/#last-modified-response-header:

New implementations should not return this header if TTL is not known; providing a static expiration window in Cache-Control is easier to reason about than cache expiration based on the fuzzy “heuristic freshness”.