The majority of APIs today are based on the Hypertext Transfer Protocol (HTTP). HTTP has been around for a very long time, in computer terms. It is the protocol underlying the Web and has changed surprisingly little since its inception in 1989. HTTP/1.1 was first standardized in 1997 and since then has been updated, but it was never replaced.
HTTP/2 was released in 2015 but did not change the way HTTP works for HTTP users. The update made the protocol more efficient by changing the way in which HTTP servers and clients communicate. However, HTTP/2 still had some shortcomings which were largely caused by the protocol using the Transmission Control Protocol (TCP) as its foundation.