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.