I noticed the AppOutlet repo has been archived, but I really liked the idea of one store to rule them all (I.e. snaps, appimages, flatpaks, etc. in one place).
Any current recommendations for an alternative?
Ah yes, the most misused xkcd. AppOutlet isn't a new standard. It's a frontend which attempts to support all of the existing standards. There is no special AppOutlet package format or repository. It's simply an application that can install Snaps, AppImages and Flatpaks, which you would be installing anyway through other means.
This is like looking at VLC's support for dozens of multimedia formats and calling it a new standard. VLC isn't a multimedia format, it's a multimedia player. It implements the existing multimedia standards, it isn't itself supposed to be one.
KDE Discover supports Snap, Flatpak and distro packages (through PackageKit). It doesn't support AppImage, though. It also doesn't work very well outside of Plasma.