Everything I have found says to go to setup>objects>Custom Object>Search Layouts> list view layout to add buttons to the list view page of a custom object in lightning experience. I have confirmed with my org and another org that the list view layout does not appear here and the only one is the search layout. Has anyone recently added a custom button to a list view in lightning?
Are you adding a standard button or a custom button? There might be no customizable standard buttons for the object you are trying to add it to.