There is an alternative to plug-ins, explained here:
COG has its own save systems that they implement before publishing, which usually fix any problems with authors wanting to save and restore checkpoints. Zombie Exodus, for example, has a save point at the start of any chapter; you can replay from the start of one and it will remember all of your stats as of that point. All of that is supported.
If people start writing their own scripts, not in ChoiceScript, to introduce to higher levels of programming - it could create unintended problems with the operation of ChoiceScript, so it's highly discouraged. Usually, people code their save states with a script or use plug-ins, then remove that when COG builds in the reliable save framework.
You don't have to. For example, some games depend on a password to unlock content or give the players access to "New Game Plus". All of that can be coded in CS, and can add some fun versatility to what you can do with save states. (New Game Plus can't reliably be coded in without a password, since there is no surefire way of maintaining the basic saved variables if people shut their browser and/or delete cookies.)
There are solutions to that, stay calm! First, you could provide your own hosting for variables to be called and remembered. Second, the ChoiceScript save checkpoints that will be added are pretty fool-proof, and allow for multiple save points. Third, you can relate things that ought to be remembered to achievements. If New Game+ is unlocked after you achieve "finished the game", it is PRETTY reliable to call back to these. It's not 100% but it will usually work.
The save system used works reliably; play a few App Store COGs and you'll see what I mean. If you quit out or your phone shuts off, it will restore your place in the story and stats as of that point. Authors can work with COG to allow for functions other than "save the last place I left off", like the chapter checkpoints in ZE.