As someone who has been involved in the deployment of network gear into class E space (extensively, for our own internal reasons, which doesn't preclude public use of class E), "largely supported" != "universally supported".
There remains hardware devices that blackhole class E traffic, for which there is no fix. https://seclists.org/nanog/2021/Nov/272 is where I list one of them. There are many, many other devices where we have seen interesting behavior, some of which has been fixed, some of which has not.
And I am sure you would agree that un-reserving a decade ago would have more than likely resulted in a greatly improved situation now. Along the lines that doing so now could still result in a greatly improved situation a decade hence. Should we still need it.
It may well have helped (a decade ago) past-tense, but it isn't the reality of today. I've pointed out there is a non-zero number of existing devices, OSs, things baked into silicon, even widely used BGP stacks today, that can't currently use class E, and some of them will never be able to. You seem to be suggesting that class E could be opened up as valid public IPv4 space. My experience is that it would not be usable public IPv4 address space any time soon, if ever. I'm not arguing that unreserving it today may address some of that. But it will never address all of it. cheers, lincoln.