Before the current regional Internet registry system (ARIN, RIPE, LACNIC, AfriNIC, APNIC) began, organizations (and some individuals) were "given" IP addresses by emailing IANA or Jon Postel directly.

Assignments that predate the RIRs are called "legacy" assignments and are, theoretically, not subject to the RIR system because those who received those addresses only agreed to the terms as they existed at the time. Those terms were usually "you asked, here you go."

In practice, legacy assignments are left alone because no one wants to go to the trouble of arguing with big entities about it. (Most of the /8 assignments people gripe about as being wasteful are assigned to entities with lawyers, guns, or both.) People who have a handful of small legacy assignments get the protection of this because it's especially not worth the effort to say "well, if all you have is a /22 legacy, yours is now part of the RIR system, deal with it". Especially since the only recourse would be to allocate it to someone else and wouldn't that be fun.

But no IP address is actually "unrevokable." All you have to do is piss off a handful of the Tier 1s or a slightly larger number of Tier 2s and you'll quickly find your "bulletproof" addresses quite useless.