We use them in AWS as IPv6 hasn't been feasible, though its become better in the last 1-2 years. AWS uses it internally, likely other big providers probably also do. If you can get away with IPv6 only, ie not constrained by provider/device, its likely worth going that direction instead.

Works fine in Linux, doesn't really work on Windows and some BSDs.

https://blog.benjojo.co.uk/post/class-e-addresses-in-the-rea...