Project Notes have been removed from the Inspector. Wait, what? I know. Deep breath. I use project notes frequently, so this is going to be a big adjustment for me.
Scrivener now has a new way to deal with them that is intended to give you broader flexibility and quick access. See what you think and learn more in the Project Bookmarks Upgrades and Project Notes Overhaul sections.
Fingers crossed that once we get used to it, the new way will be better.
P.S. If you open a Scrivener 2 project in Scrivener 3, the update process will move your project notes to documents in a Project Notes folder in the Binder, as shown below.