While it was introduced in 2015, the adoption of the technology is slow. HTTP/2 builds on the HTTP/1.1 protocol to allow for various features that caused issues for the previous system. This gives us the ability to use a single TCP connection to receive different requests. This wasn't possible with HTTP/1.1 and it caused an issue called head of line blocking. This meant that we could only really handle so many TCP connections and that if we had a long-running TCP connection, it could block all of the requests after it.
HTTP/2 also gave us the ability to push server-side resources. This means that if a server knows that a resource is going to be needed by a browser, such as a CSS file, it could push it to the server before it was needed. ...
