Getting a "portable" /19 or /20

In the referenced message, Eric A. Hall said:

Also, they don't have any special-case handling that I am aware of. I
tried to get a private /24 to use for the topology examples in my books
and couldn't get one. ARIN outright refused the request even though I
could prove the need for it, and even though I didn't care about global
routing or reachability.

Internet Assigned Numbers Authority (NET-TEST) IANA -

The above, iirc, is the appropriate netblock to use in documentation
examples. So, they have already assigned you and every other author in the
world a /24 to use.

To be pedantic, IANA has, ARIN didn't.