Never.
Like IPv6, HTTP/2 is a broken standard that doesn't solve any problems.
The problem with the Web isn't a lack of out-of-order requests, the problem with the Web are those horrible ever-present Javascript frameworks.
(Likewise, the problem with IPv4 isn't a lack of address space, the problem with IPv4 is a lack of a standard and simple way for managing VPNs and tunnels. I don't want my lightbulbs to be connected to the global Internet. What I _do_ want is a way to tunnel remotely and securely into my home's LAN and control the lightbulbs from the inside.)