This was something that Google should have added before the first Chrome OS tablet arrived. There was a flag available in chrome://flags, but nothing was functional. Now, a few weeks after the release of the first tablet, lock screen notifications are finally working on Chrome OS.
This has been around for a while now:
Name: Lock screen notification
Description: Enable notifications on the lock screen. – Chrome OS
This seems to be working on the latest Canary channel update. This, however, does not look like what I had expected it to be. Instead of showing the notification content on the lock screen similar to Android, Chrome OS shows the number of notifications in the system tray. You will see the list of notifications when you click that icon.
I would hope that this works differently (similar to Android that is) on tablets. My Pixelbook in tablet mode, however, has the same behavior.
The team is still working on this. There was a new commit today for the lock screen notification feature:
Initial implementation of Lock Screen Notification
This CL introduces the feature to defer events from notifications until
the device unlocks, add adds the UI to encourage user to unlock the
clicked notification to perform their events.
The main concept of this CL is introducing LockScreenManager to message
center code, which intercepts the event handlers in MessageCenter and
defers the event if necessary. And Ash has the implementation of
LockScreenManager named AshMessageCenterLockScreenManager, which handles
message center events depending on the status of the lock screen.
I will keep an eye on this feature as this develops further. Hopefully we will see lock screen notifications similar to that of Android.