There are two interwoven threads of logic in my post. You've mistaken the second as limited to the first when it is building upon it.

To clarify:

It /used/ to be that .local wasn't mentioned in any standard.

Don't do anything like .local (a non TLD and not part of a standard), because there is now a history of standards being created that break existing private deployments, and also because OTHER TLD like things can now be purchased as TLDs. That is why the current best practice is now to have an internal subdomain of a real domain (even if not globally published).

this should be the rfc: https://tools.ietf.org/html/rfc6762

from wikipedia:

> The Internet Engineering Task Force (IETF) standards-track RFC 6762 (February 20, 2013) reserves the use of the domain name label local as a pseudo-top-level domain for hostnames in local area networks that can be resolved via the Multicast DNS name resolution protocol.[1] Any DNS query for a name ending with the label local must be sent to the mDNS IPv4 link-local multicast address 224.0.0.251, or its IPv6 equivalent FF02::FB. Domain name ending in local, may be resolved concurrently via other mechanisms, e.g., unicast DNS.