etlegacy VS dxx-rebirth

You could try Dirty Bomb or the classic Wolfenstein: Enemy Territory for some class-based objective shooters. Both are free to play.

ET Legacy Wolfenstein Enemy Territory, FOSS client. Maybe moot since an official client re-released on Steam recently.

It doesn't replace any server, it's a new one on Legacy mod - https://www.etlegacy.com/

Look up ETLegacy some nice guys from the old scene have basically remastered it - hd graphics updated sounds etc. All things in one place!

ET: Legacy

Have you tried using ET Legacy? www.etlegacy.com It's basically the same ET client, but updated to current standards.

As you pointed you need source code that can be found here: https://github.com/etlegacy/etlegacy

Off topic, but if it was Wolfenstein ET I will just add that there still is a community for it and a Foss version of it that runs on modern operating systems

https://www.etlegacy.com/

And that's the first level of Descent. :) I literally just started a brand-new game yesterday of Descent. DX-Rebirth.

Find a remake of the game ported to a modern engine. https://www.dxx-rebirth.com/ appears to be a port of Descent 1 and 2 to the Source engine. It won't help with you running the 3DFX version, of course, but this might actually be better?

I loved this game. You can still play it with DXX Rebirth

I would look at DXX rebirth, https://www.dxx-rebirth.com even if you just want to add the expansions to your steam version, you should be able to use its instructions to add the levels to your version.

Descent 2 - so long since I last played any Descent game I forgot how the experience was, but hey there's a source port!

Or if you still want to play the originals, the DXX Rebirth mods also make it easy to play on modern computers!