https://www.opengroup.org//openbrand/register/

To actually pass the certification test suite on a real system, Apple sometimes needs to apply special configurations (e.g., disabling System Integrity Protection (SIP), using case-sensitive filesystem, enabling certain legacy services, etc.).

telnet(1) is not required by POSIX (nor is nc or ssh required!)

Ironically, telnet(1) did not begin as a "Unix" utility but an ARPANET protocol suite program. It was available cross-platform. It is unclear whether all editions of Unix included a client, but BSD for sure was the point where telnet and TCP/IP became essential integrations for the systems.