The last time I looked into it, a few years ago, everyone except Apple had some basic amount of support. (That is, openssl, firefox, windows, and android — that's all I looked into.) I set up my private CA with name constraints, but I had to mark them noncritical if I wanted iDevices to be able to use the CA at all.

But the other systems did at least appear to honor the constraints.

I've asked Apple security folks about this a few times, but they just look uncomfortable and change the topic.