Re: Future use??
You'd be surprised by how much of the RFC1918 address space does get advertised for routing over ISP networks, which is a potential vulnerability if someone tries to connect to it and ends up reaching a malicious server. Most devices SHOULDN'T be handling packets for those blocks outside their WAN generally, and never advertising routes via routing protocols, but there's technically nothing that says you can't use a router internally at multiple points where such routing would be valid so outright preventing it wouldn't be good and it comes down to either bad configurations or malicious configurations. But 240/4 being reserved as a "never use this" block makes it a bigger deal to prevent anything on the public Internet ever accepting routes or sending traffic for it. Kind of an arbitrary dividing line between degrees of risk, I guess. But after we reached the point of not having anymore IPv4 addresses, and deciding that IPv6 was the future, 240/4 should have been released as there was clearly never going to be any other "future use" or experiments. (And what did they need 168 million addresses for when experimenting?)