In Photos, how can I auto-split events upon import?

Solution 1:

Events are deprecated. Your library is just shown in an interface with date & location (if available). If you want further organization you can (have to) create albums.

Solution 2:

If you haven't already, turn on the sidebar (View menu > Show Sidebar). At the top of the sidebar is a button labeled Photos. That will show you all of your photos divided by date (multiple "events", or photo groupings, for each date will appear if there is more than one location involved). Beyond that, no, there is no feature directly comparable to Events in iPhoto, though you can create albums if you want to combine multiple "events".