Folder view toolbar (located at bottom) has been replaced by inline links and context menus. Along with this visual change the names of the corresponding extension had to be renamed.

<app-id>/folderview/sidepanel/toolbar/options is now
<app-id>/folderview/sidepanel/context-menu

<app-id>/folderview/sidepanel/toolbar/add is now
<app-id>/folderview/sidepanel/links

Examples:
io.ox/mail/folderview/sidepanel/links
io.ox/calendar/folderview/sidepanel/links