I believe it was made with Twine:

https://www.konbini.com/en/techandinnovation/twine-free-tool...

http://twinery.org/

They made the first prototypes/scripts for Bandersnatch in Twine, but they later moved to a Netflix-internal tool called "Branch Manager".

Cool, thank you both.

Inform 7 is interesting, but I never really liked coding in it; I feel like its almost-natural-English-but—not-quite syntax falls into a sort of “uncanny valley” of programming. Because of how much like natural language it is, it always feels like you should be able to write things in a lot more ways than you actually can.

Sounds a bit like AppleScript.