Like any Internet Explorer on any non-Windows 10 systems?

If 47% of all CloudFlare users are not connecting via HTTP/2 (even though it is supported server side), I believe it means that those users do not have client side support (or are behind a proxy, etc. which doesn't allow it).

At the end, if someone is thinking about making a website which would be much slower for non-HTTP/2 users then right now it means around 47% of their users.