I just bought and installed the game. At about 75% install it asked if I want to download and install Beta. I generally avoid Beta programs due to stability concerns, so I declined thinking I could look into it later and make an informed decision.

I forget what the Beta description was and didn't write it down because I assumed it would be available in GOG or in the game.

In the GOG program, it says there are no updates available, and no mention of the Beta.

I ran the game and looked through all the settings. I can't see any mention of the Beta there either.

So now I can't find it and Google search came up empty. I guess I could uninstall it, and then install it again, but much simpler to ask here.

What is the Beta program for and what does it do? Is it stable and should I install it? And where can I find it?

No clue, sorry... because i did not get such a "invitation". Alpha or Beta tests are usually closed and only a "chosen few"... perhaps randomly, may be chosen for participation. However, i lack any infos and most likely this option is now gone.

The dev team most likely may not answer you on GoG and perhaps not even on Steam, as they are simply to busy with all of the new content they are currently creating. Finally the full "new content" is now available on GoG, even the supporter pack.