I think there was an initial release that was free, but they later rereleased with additional content and started charging. Hence the “Plus” in the name.
Ahh, makes sense. Because I distinctly remember just downloading it on a whim when it was new and going in completely blind, save for the cryptic "not for the faint of heart" warning. Absolutely wild ride, but seems like it kinda blows now that it's extremely well known what the game is