Message #5 from the Steam community forums ("roll back" Windows 11 to before the 24H2 update installed) apparently only works for a limited time after 24H2 has installed. The immediate problem with Message #18 from that forum is that the Group Policy Editor is not natively included in Windows 11 Home Edition. Message #18's poster gives instructions for editing the registry directly; however, I chose to Google the error message I had received ("gpedit not found"). There were many suggestions online; I found one which involved downloading a batch file (Microsoft will warn you to only download from trusted sources -- so I certainly hope this was a trusted source!), letting the batch file "run as administrator" to install, then restarting the computer. I have just installed that batch file, and am about to shut down my computer for the night. In the morning, I plan to follow the instructions from the Steam forums post to edit the group policy so that Windows 11 will revert to 23H2 and not attempt to reinstall 24H2. Then we'll see if I can finally run Alien Crossfire again!

Finished following the instructions from Message #18 in the Steam forums on this topic, to edit the Windows 11 group policy so that I'd get security updates for the Windows 11 23H2 feature pack without the Windows 11 24H2 feature pack reinstalling. Alien Crossfire is not yet successfully running after restart, although Message #18's poster did say that it could take several hours before the new group policy would take effect. I might try waiting 24 hours (until approx. 6:45AM in my time zone tomorrow morning) and try loading SMACX again.
(Should I perhaps have tried uninstalling the Windows 11 24H2 feature pack, too? It's been too long since Windows Update installed it to use "roll back" to get rid of it easily, and I'm not quite sure how to accomplish that otherwise, so I'd have to research how to do it.)

Is there a way to run SMACX on win11 without blocking the current 24H2 update? From a security perspective, it doesn't seem reasonable to me.

I don't want to install a virtual machine because of this either.

Is there a chance that GOG or Steam would release a patch that would allow playing on current win11?

The game works without the Crossfire expansion. I wanted to play and add the SMAC 444 AI Patch. With that, the AI ​​wouldn't have to be so bad, unfortunately I couldn't find the patch anywhere. Do you know if there is a mirror somewhere to download?

It appears that enabling Group Policy Edit and having it tell Windows 11 to only install security updates for the 23H2 feature pack does NOT get rid of the 24H2 feature pack; it just prevents the 24H2 feature pack from being reinstalled after one has gone back to 23H2 by other means. For those of us who didn't think to "roll back" to 23H2 immediately after 24H2 was installed, it sounds like one would have to reinstall Windows 11. There are YouTube videos describing how to do this, but the process sounds quite complicated and would probably involve reinstalling other software as well.
I would second @hovno666x's wish that GOG or Steam would just release a patch for the Sid Meier's Alpha Centauri Planetary Pack which would allow Alien Crossfire to run properly on Windows 11.

Considering how software companies run nowadays: im in before, gove us money we let you play a decades old game again.

Might I suggest to (pity the fool) and simply use the Windows Linux Subsystem to dole out a healthy does of wine to run these games by?

It's how I've been playing SMAC on Linux for years because I frankly couldn't figure out how to get the Loki based executables running.

./play.it can handle that for you, but that would require adding support for RPM packages first: https://forge.dotslashplay.it/play.it/play.it/-/issues/3

I got positive feedback from people generating .deb packages then converting them to .rpm using alien, but this does not feel really satisfying.

Anyone who is willing to play a preview development build of the Thinker mod can try the new version that just came out, detailed here. It contains a fix for the bug. I haven't had time to actually test anywhere near a full game, but it seems to work for me so far.

Yep, it works. Grab the latest development build (20250417 works at the time of writing), extract to game folder and start the game with thinker.exe

I can also conform that it works. Thinker-dev-20250421 not only solves the Win 11 bug, but it also improves certain gameplay issues and makes me wonder why I had not tried the Thinker mod previously! ;-)

The issue seems to be the games save file code relying on the value of some unitialised memory, which the latest version of Windows changes

I've got a patch for the base game file which seems to work. I've tested on Win11 24H2 and Win10. However I've never played the game before so my testing has been limited

Looking at the fix in Thinker I would say that is better than mine, but if anyone is interested in the vanilla exe patch, or just some more details I'm happy to share

I've also contact GoG directly to see if this is of any use to them

Just downloaded the latest developer build of the Thinker mod (20250429) and extracted to the Planetary Pack game folder. Now I'm seeing a red "down arrow" Downloads prompt on the game page in my computer's install of GOG Galaxy, and an error message "Update failed (E3): Failed to connect to GOG.com servers." There is a "Retry" link, but it's not currently clickable.
(I had previously tried the 20250417 dev build of the Thinker mod, and hadn't gotten the Downloads error message on the game page, but hadn't seen thinker.exe show up as an "additional executable" either. I clicked the main Alien Crossfire executable link (not the PRACX link) and got part of the intro movie before updates to other programs interrupted. (Dumb idea to install updates to multiple programs at the same time I was trying to fix Alien Crossfire, I know.))
Should I wait for the GOG.com servers to become available, maybe try installing the 20250421 dev build of the Thinker mod, or something else? Perhaps I can also try contacting GOG.com support again, since apparently there are multiple reports of the latest builds of the Thinker mod fixing the Windows 11 24H2 crash, so GOG.com support may already have helped others trying this fix.

Nathan Baggs kindly shared his fix with GOG directly. Crossing fingers they'll review and patch the issue.

More detail on his YouTube channel (just posted), but I'm not cool enough to post links yet.

But does Nathan Baggs' fix work with the unofficial patches? The last time I played (quite some years back now) I was using Scient's unofficial patch (v1.0). Doesn't look like that's been updated for some time, but I can see a v2.1 fork from 2019 on Github here: https://github.com/DrazharLn/scient-unofficial-smacx-patch/releases/tag/v2.1

If Thinker has the same fixes, plus some other QoL changes, then maybe I'll go with that the next time I play.

Ya seen that video aswell :)
Hopefully the Exe will get patched.

I dont want any third party mods :)