I was curious to see more details (such as the the version) of the chromium snap using the gnome-3-28 extension
(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.
Branch, thanks, I thought channel wasn’t right. That makes sense for the bare snap info command.
But if I know the branch name, it would be great to be able to see details of what you are going to install with a command like
snap info chromium --branch=candidate/gnome3-28-extension