Actually that is not true. The IETF has expanded the definition of “TTL” and explicitly permits resolvers to serve “stale” RRs beyond their expiration time.

https://www.rfc-editor.org/info/rfc8767/

As a corollary, there is obviously no floor on refetching unexpired RRs, of course, except for efficiency concerns.

That's only when the authoritative server cant be reached though