This is a really bad change. We have 7 different group inboxes that we use, and different staff that have access to different ones in our project.
While the “My messages” is nice (kind of?), why also remove the ability to have the inbox list? Why not at least make it a toggle?
Can a toggle be made to “restore legacy inbox section”?
We won’t turn this into a toggle most likely.
What you could do as a workaround, is just making a custom section in the sidebar and add direct links to all the 7 checkboxes.
How can we make this visible to all the groups for those users?
I think the downsides to that is there’s no guardian to show only those group inboxes that each member is permitted to see, and I don’t think the custom sidebar links show any unread counts (Display number of unread messages on links to group inbox in sidebar)
Yeah, its not a perfect solution 
But better than nothing for their specific usecase
I suppose encouraging each member to create their own private custom section and populate it with their inboxes may get you slightly closer. 
But if this is not going to be picked up as a core feature what would be the best way to build a customisation to achieve it? Would it need to be a plugin, or a maybe even just a theme component? Or is it just not possible?
Other downside is that the links do not show the number of new messages 
Was that’s another thing. Seeing the number of messages was a huge benefit that is no longer around.
What a shame.
I don’t want to raise too much hope here, but I think adding a sidebar section with the group inboxes including unread indicators is possible with a Customization > Theme component. At least I got this:
See
