To all those wondering about updates - it is still under development - I know as I am a contributor.

https://github.com/team-inbox/inbox-reborn

This is a godsend. Absolutely beautiful. Unfortunately, it hasn't been updated in a while, and the Compose button displays incorrect: the red circle on the bottom right now doesn't open new emails, and there's a hovering compose button in the top left that doesn't display correctly. Fingers crossed that an update will come soon. Thanks so much to the developer.

The best substitude for fallen Inbox by Google I have found (and I have been looking for alternatives for months).
This add-on is simple yet effective and actually contains "Bundles", which is very rare.
Excellent & intant recommendation! Great work!!! Thank you

Very impressive. Immediately less stressful trying to make sense of messy threads etc.