Well, that's weird. I never dabbled here before but wanted to try the game with all the content available, so I don't really understand that convoluted paywall bs, but it seems like no matter what the version, you can find the extreme code in commonevents.json (like some people here seem to do with mods/forced unlocks), but even by using the "correct" code for the public version available in the OP, it just didn't work.
After downloading the 1.77.6 from the link a little bit above, the code written in the files does seem to work. After checking the other version, it seems like what is usually the "enabled extreme content!" popup is instead the "wrong code" message, with the latter existing twice.
So, is the public version completely unable to enable extreme content despite having the option and the code for it inside, or did the dev unironically used a "wrong code" message despite said code still working? Because according to a very long chain of patreon comments under the (sadly seemingly outdated) extreme code post on kemono claiming that the good codes never worked for a looooong time, the latter seems plausible, but at the same time this weird paywalling and versioning dynamic makes the former plausible too...