Often things are allocated and remain unused, but cannot be reallocated. So companies end up with large parts of 10/8 reserved. I recently had to start using some weird 100 addresses (some reserved router protocol) for this reason. It doesn't help that a lot of AWS stuff wants to consume an IPv4 for no good reason (IE, the machine gets an IPv4 but isn't reachable from the internet).
100.64.0.0/10 is reserved for CGNAT providers to assign to CPE. If you don't connect to a CGNAT then there's nothing stopping you from using it, however. Tailscale hands out addresses in 100.100 for that reason (deconflict with customer private networks)
