Well I feel like an idiot. I thought the quest everyone was referring to was the rope quest, and that the slime girl was supposed to be in that room during it, not a separate quest afterwards.
I just tested it, and it seems to be both broken and available because I did the chapter end quest.
I can get the quest, and go fight the slimegirl, but the fight is broken when I do, because no one can be actually stripped, their defeated images don't appear if they lose, hitting zero resistance does nothing, and there is no game over loss screen, just the standard game over message and back to the main menu.
So I guess I just have to load up one of my older saves and use that, unless that was just the state of the slime girl as of the final demo, and she's only done in the Patreon version.