I use Discover on Kubuntu 17.10 with the following PPAs installed: - Kubuntu backports (provides newer versions of KDE software) - VLC nightly builds (provides a newer version of VLC) These PPAs provide versions of their software that have AppStream IDs identical to the versions provided by the regular Kubuntu 17.10 repos. I know that Discover has a backend switcher UI: https://www.youtube.com/watch?v=qzOZZvabNeg&spfreload=10 However, Discover doesn't show the backend switcher on the Application Pages of any apps that are available from both a PPA and the distro packaging. Instead, the PPA version is always shown and the distro-provided version is unavailable until you manually disable the PPA.
I'd say PPAs are deprecated. I prefer spending time in other stuff.
Okay, fair enough.