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

Version 1.1 has been released

v1.1 Latest @arbruijn arbruijn released this 25 Jan 10:18 v1.1 […]
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v1.1 Latest
@arbruijn arbruijn released this 25 Jan 10:18
v1.1

Note for linux/mac users: the default game folder now uses the name redux instead of rebirth, you probably need to copy/rename it.

General:

Automatic demo improvements
Allow using joystick axis as buttons
Added Classic No Ammo Autoselect option
Allow multi-character search on listboxes
linux/mac directories now have the name redux
linux support for .ini file in .AppImage dir
Added -gamelog_split option
Improved png texture loading time
Fixed sound exactly from right playing on left
Fixed hostage door after aborting while it explodes
Fixed playback of multi-level demos
Demo lock sound/indicator fixes
Now creates crash dump files on crash
"Mouse Smoothing" name change
Fixed cockpit appearing after exiting while dead

Multiplayer:

Added Only Show Confirmed Hit Sparks option
Removed DXX-Retro Homing Speed option
Better synchronized homer scanning interval
Fixed bomb death spew with extra/capped weapons
With friendly fire off, also ignore friendly splash damage

Observer mode:

Observer mode demo fixes
Added third person, muzzle observer options
Fixed disabling Show Bomb Times
Fixed observer afterburner charge indicator
Avoids overlapping kill graph and monitor views
Better support for switching cockpit mode
Fixed rear view monitor

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