A simple $ route1_cleared = True and $ route2_cleared = True variable placed at the end of each route would allow you to make the third route appear after they cleared both routes easily.
Yes, but... then the player has to beat both routes on the same save file, before that third route would pop up. And it'd only pop up on that save file - starting a new game wouldn't show the third route.
By XYZ you mean the result of the coding?
No, by XYZ I mean whatever variable it is you need to check, not the result of the code itself. Do you need to check if the player has already seen this route? Do you need to check if affection points are high enough to unlock the other menu option? Things like that.
And I found this code called MultiGame.Persistence. Do you think I can use it for making a lockable character story?
From the documentation: "Multi-Game persistence is a feature that lets you share information
between Ren'Py games."
As in, a completely different game. A different project. Like if you wanted to add in a little bonus if someone played another one of your games already, that'd be something you would use.
And also I forgot to ask. What is .org and how can you get it? Is that different than .rpy?
I'm pretty sure "demo.renpy.org" used in the example is just an example of a string, rather than a specific file. Renpy.org is the official website for Renpy, after all, and I think demo.renpy.org may have been what games.renpy.org is now.