Demand for multicast addresses was grossly over-estimated and no
viable new addressing mode for class-e ever appeared.

https://tools.ietf.org/html/rfc1112#section-4

1981: Class E IP addresses, i.e.,
   those with "1111" as their high-order four bits, are reserved for
   future addressing modes.

In the interim, since the last attempts at making class-e addresses
work, for some purpose (), most operating systems and stacks chose
to treat 240/4 as globally reachable unicast, and those that didn't
(windows only, so far as we have found), not allow them at all.

* Fixing Windows

Will probably take an RFC