It looks like this will win: https://en.wikipedia.org/wiki/.internal

example.com, and the reserved TLD ".example", exist for technical documentation and writing. If you are writing a comment on HN, or a curriculum for a networking class, then you can discuss "foo.example.com connects to bar.example.com" or "Let's hypothesize about two offices called accounts.example and human-resources.example"

The "example" domains are never supposed to reflect anything that is actually deployed onto LANs, or test labs, or the Internet, current situation notwithstanding.

https://en.wikipedia.org/wiki/.example

There are, likewise, IPv4 and IPv6 ranges that are reserved to be used in documentation. Not the 192.168.0.0/24 or 10.0.0.0/8, but separate ranges that writers only write about, and are never deployed, not even in private.

localhost is only ever going to be the loopback interface, never across a network: https://en.wikipedia.org/wiki/.localhost#Conventional_use

See also: https://en.wikipedia.org/wiki/.test

The latter article lists foreign-language TLDs which serve the same purpose.

Some proposals are described here: https://en.wikipedia.org/wiki/.home

And there's also .local for mDNS on local network!

I've also come across projects using a public DNS record that points to 127.0.0.1 (something like localtest.me?). IMO that's way worse than using .localhost since you're trusting some rando not to change the DNS records and exfiltrate your meant-to-be-local traffic.