Multicast just doesn’t seem to be happening at all. Even AA, one of the very geekiest of ISPs, does not support it although they have talked about it, but no one is asking for it anyway. There’s a vast amount of space to be reclaimed at 240.0.0.0/4 - how much could be reclaimed and how much work would it be? Does it involve fixes to operating systems at all?
I have firewalled off almost all of the addresses in the RFC5735 summary table such that tx to those addresses is blocked as well as rx, although rx is not an issue because of the usual stateful firewall ‘Dracula and the maiden at the window’ default firewalling rule. Ditto for IPv6 too.
A couple of questions: how many ISPs block upstream traffic destined for the evil RFC5735 dest addresses? Also do ISPs block bogus traffic with a source address in the evil RFC5735 address ranges, be it upstream or downstream?
You could perhaps test it for upstream, but downstream might be more awkward unless you have your own server hosted somewhere in the internet, and even then responsible anti-spoofing and anti-bogon filtering might ruin the experiment.