The game doesn't need that feature, because Ren'py allows the player to save in every moment. I suggest to add the disclaimer "End of the update, not of the game dumbass!" in the panels which says "Shannon ending", "Angel ending"…I'm honestly torn a little between setting up a screen that tells people to save at the point before they make a decision and explain that people are simply making a decision about who they want to end the night with and not saying anything.
Maybe the word "dumbass" is avoidable.