Apple deprecated QuickTime in favor of AVFoundation. They have dropped support for QuickTime on Windows already, and they have announced that they will drop support for all 32-bit apps and frameworks (which includes QuickTime) with the next version of macOS. Developers who were dependent on QuickTime have been forced to rewrite their media-handling libraries.
As far as I know, it is not possible to make a user-facing codec plugin for AVFoundation like it was for QuickTime. I think that the AVFoundation Hap support you mention is a developer-facing framework to allow apps to use Hap-encoded media through AVFoundation, so Hap support must be specifically written into the application.