Large companies misuse "unassigned" space all the time. I have heard engineers at my work propose using the non public routed DOD /8 before. Not on my watch!
If you happen to know why, could you explain their reasoning? In what ways are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 insufficient?
I help design one of big 3 cloud providers and we're about to run out of private space for customer IPv4. We are addressing this is in a number of ways but I think others have run into this same issue.
And I can confirm that this is done at some Very Large ISPs. Seen it with my own eyes, e.g. 30/8