Yep. RFC2606 It is what they should use. And if you need to specify 2 hosts, you can use example.net and .org as well.
Unfortunately, the example domains don't convey context very well, so we see things like target.com, victim.com, etc
This can be corrected by target.example.com and victim.example.com. Conveys the context while remaining safe as an example.
That generally works, although in some cases it makes a difference whether two hosts are on the same tld; at the very least, it implies a connection between the two that may not always make sense (why is aggressor.example.com attacking victim.example.com?).
The same goes for TEST-NET (192.0.2.0/24), TEST-NET-2 (198.51.100.0/24), TEST-NET-3 (203.0.113.0/24), MCAST-TEST-NET (233.252.0.0/24), and the IPv6 documentation-only prefix (2001:db8::/32).