New Demo:

https://www.gog.com/en/game/bad_cheese_demo

New Demo:

https://www.gog.com/en/game/dreamed_away_demo

Rosewater Demo
https://www.gog.com/en/game/rosewater_demo

It was not announced at the time of the full game's release
https://www.gog.com/news/release_rosewater_with_soundtrack
https://www.gog.com/en/game/rosewater

Most demos become available before the full game release.

So the Rosewater Demo was released at GOG some time ago.

New Demo:

https://www.gog.com/en/game/prim_demo

I figure a mention here might be useful as well:
gamesieve.com (just announced) has a filter for "games with a demo", and if you toggle off grouping, also a filter for the "demo product type" (both using a liberal definition of "demo", including prologues and various other free "taster" products for a full game).

I'm categorizing products manually; so if you disagree with me on any categorization - or if I missed anything - I'd be happy to hear about it, and could be convinced to change my ways. :)

New Demo:

https://www.gog.com/en/game/craftcraft_fantasy_merchant_simulator_demo

New Demo:

https://www.gog.com/en/game/beacon_of_neyda_demo

About a Boy Demo
The Glitch Fairy Demo

Readded and updated Demo:

https://www.gog.com/en/game/becastled_demo
https://www.gogdb.org/product/1539835574#changelog

Is there a thread in this forum about

"Demos, Prologues, etc. that have content not found in the main game"?

I remember, there was such a demo for "The Monkey Island" (a long, long time ago). :)

An extension of this thread would be beneficial.

Be handy to have that, but though I have seen such comments in passing, no specific thread about it exists to my knowledge.

Sometimes that info is mentioned on the Demo page or in the forum section for the game and maybe externally elsewhere (i.e Wiki etc).

For sure, a bunch of demos do have differences or a map or level that ends up being left out of the full game.

Useful links:

Discussion about Demos on GOG:

https://www.gog.com/forum/general/the_discussion_about_demos_on_gogcom

Place where you can publish a job offer:

https://nofluffjobs.com

Thanks :)