BuddyPress.org

Lest anyone stumbles on this sad, lonely, and un-answered post looking for the same slugs. Here’s the update:

Unfortunately, the answer appears to be “no, there are no slug constants at present for the subnav items such as inbox”.

It’s covered towards the end of this page: https://codex.buddypress.org/buddypress-site-administration/customizing-labels-messages-and-urls/