The full error message is:
Symfony\Component\Routing\Exception\RouteNotFoundException:Searching through the database, I found a row in table shortcut that referred to that route. After deleting the row, the site worked fine.
Route "optimizely.add_update" does not exist.
(line 150 of core\lib\Drupal\Core\Routing\RouteProvider.php).
I don't remember creating such a shortcut - I didn't even know about shortcuts before this problem happened - and don't know how it came into being.
But I was able to reproduce this by manually adding a shortcut for the path to one of the Optimizely tabs, then uninstalling the module. The same error message came up.
I haven't found any recourse other than to go into the database to delete the shortcut from the shortcut table.
The first article below reports this issue along with a recent attempt to fix this in core.
RouteNotFoundException when a module (previously added to shortcuts) disabled
Working with the shortcut bar