What about IPv4 class E?

Support.

Networking stack implementations are written to support the RFC, and hence will not sent or accept packets from IPs that are reserved for future use. Besides the networking stack, services have to support them as well; DHCP has to be able to distribute them, DNS has to be able to store them, the software at IANA and your ISP must be able to actually support the creation and usage of that block.

By the time we all support this "class E" block, we will have made big progress switching to IPv6 so it'll no longer be worth it. Developers, ISPs and consumers better invest in switching to IPv6 instead...

However, many TCP/IP stacks, such as the one in Windows, do not accept addresses from class E space and will not even communicate with correspondents holding those addresses. It is probably too late now to change this behavior on the installed base before the address space would be needed

— Read more (under Address Reclamation a Solution?)

The problem is that this block is blacklisted in many operating systems. So they won't accept it as their own address and they won't connect to servers with such an address.

The other reason is that using class E would only extend the lifetime of IPv4 a year or so. Upgrading all operating systems for a year of extra delay with deploying IPv6 is not worth it