I have already lost track of all the Descent port flavours, how is this one different from DXX-Rebirth?

I was wondering the same thing. This is just speculation, but based on some of the information on the DXX-Retro GitHub, I would say that Arne de Bruijn may have decided it was easier to just fork DXX-Retro rather than struggle to get DXX-Retro updated with fixes. It has been almost six years since any new official DXX-Retro versions.

If you really want to know, take advantage of the Discord channel given on the DXX-Redux page.

I have used the D2X-XL for years.
https://www.descent2.de/
But it's always good to have other options.

Version 1.0 has been released.

v1.0 Latest […]
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v1.0 Latest

Supports separate observer options for each game mode
Added observer option to show bomb timeouts
Combine laser levels in observer damage lists
Supress kill/death messages if the observer death log is active
Added option to disable chat sounds
Extended Abort Game menu
Fixed observer crash when player hit by reactor
Fixed auto demo recording in D2
Fixed keyboard slide left sensitivity setting
Improved Linux package (now works on the Steam Deck)

Downloads: https://github.com/dxx-redux/dxx-redux/releases