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.
I said "all current, major browsers" in my original post. IE is not Microsoft's current browser. In addition, not all web sites are serving via http/2 and that web sites aren't using http/2 is no fault of the browser or the user.
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.