So, in speedruns I saw you can do a fun thing where you get the local villagers to kill the final boss and complete the game in minutes. But apparently this got patched out, I hear.

Is it possible to downpatch the GoG version to that older version?

You can try downloading and installing the old patches (patch scrolls, etc.).

https://www.patches-scrolls.com/two_worlds.php

For versions 1.5, 1.6, and 1.7, you can unzip the installation patch file with 7zip (looks like an nsis installer). Previous versions must be installed (native EXE).
Make a copy of the locally installed GOG TW1 game folder as a backup.
To downgrade to 1.5, you must remove Update16.wd from the game folder and copy the 1.5 EXE from the patch archive.
The same applies to v1.6. I'm currently playing GOG version 1.7 with 1.6.
To downgrade further, you must start at patch 1.10 and then check which WD archives have changed.
The patch "Update11-15.wd" comes from the major update 1.0 to 1.7 or 17b.

Or you can watch the outro of TW1 with a movie player. There's no faster speedrun to get the Outro. :D