I’m wondering if there’s any way to make a variable automatically save its old value, so that I can, e.g., write a minor mode that will automatically restore the old values of variables when it’s deactivated (without having to declare a bunch of -old-value variables).
The function
add-variable-watcherregisters a callback that is invoked whenever the variable’s value changes. You could use it to save old values in a list or other data structure.