(emphasis added), but as far as I can find, snap info only shows the “latest” channels. Am I missing something? If not, consider it a feature suggestion
candidate/gnome3-28-extension is a branch and those are indeed not discoverable by design; one has to know the name of the branch to instal from it, it won’t show in snap info.
latest is the track component of the channel and it’s implicit here. It’s equivalent to latest/candidate/gnome3-28-extension.
Snap info will show data on other tracks and risks if they exist
You can learn more about channels and their components (track/risk/branch) here.