In 2.79 I could press U to make data single user. How can I do this in 2.80, what is the hotkey?
One way to do restore the functionality to a hotkey is to add a custom keymap in Preferences. I added one under 3D View > 3D View (Global) with
object.make_single_user as the function and Selected Objects for Type.
It seemed to work ok, but it doesn't bring up the context menu like before. The options appear in the small window at the bottom of the screen, usually collapsed by default. Or if you always want to perform the action on data, I think you can pre-check that Object Data box in the keymap settings and bypass the context box entirely.
If you want to keep the modal context menu with a hotkey though, you can right-click the Make Single User menu item (under Object > Relations) and add it to your Quick Favorites menu (Q).