The selection toggle arrows in the Outliner disappeared. Is there some preference in the settings that has to be turned on?
These are now hidden by default to provide a cleaner and easier UI for beginners while still presenting enough functionality for most common use cases.
For more complex projects or advanced users Outliner Restriction Toggles can be turned on or off and customized according to personal preference from the Filters pop-over in the Outliner header. There you can choose which restrictions columns are visible in the outliner by default.
You can still control collection visibility restrictions if the columns are off from the context menu by right-clicking an object and accessing the Visibility submenu.
4$\begingroup$ If the outliner view's space isn't wide enough, you might need to either scroll over or widen the view. $\endgroup$ Sep 2, 2019 at 4:13
$\begingroup$ this is a great example of keeping existing functionality while hiding it for beginners <3 $\endgroup$– blentedJan 8, 2021 at 2:20