Jonathan Hurwitz -- who was working on the show for Disney at the time -- hit TikTok to explain what they were penning, and what pissed the Mouse House off in terms of plot points. Basically, he says they were writing about a 30-year-old woman doing grown-folk things ... which weren't always G-rated.

Check out his highlights of things they were having Lizzie do in their comeback season -- he says Hilary Duff would've been in the full swing of NYC living ... working as an interior designer while shacking up with her handsome chef BF -- who'd cheat on her with her BFF.

In their nixed script, they had it where Lizzie then moved in with her parents in Cali ... where she'd find herself back in contact with Gordo ... who's got some major bombshells for her -- he's engaged with a baby on the way!

According to Jon, Lizzie would've received a text from her old high school crush, Ethan ... later checking him off her "to-do" list after waking up in his bed -- something he says that particularly pissed off the family-friendly folks over at Disney. No sex, dammit!

Two episodes had already been filmed before Disney+ pulled the plug following creative differences between the streamer and Hilary ... which was pretty dramatic when it happened.

Hilary announced the decision on IG in Jan 2020 ... saying she wanted the reboot to be an authentic portrayal of who Lizzie would've been today -- anything else would've been a disservice to her. Sounds like Jon here is cosigning that ... noting, Disney wasn't down.

Of course, it's devastating that adult Lizzie never got the reboot she deserved ... but nothing's more upsetting, perhaps, than learning Gordo never got with her himself after all those years.