You don’t have to declare a new variable each time you want to remember an old value. You could, e.g., put a custom symbol property on the variable symbol with the old value (or list of old values) you wanted to store. Or maintain a single variable mapping all your variable symbols to their old values.

There is buffer-local-set-state at least, not sure if it is any good.

The function add-variable-watcher registers 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.

A variable watcher maybe.

    (add-variable-watcher
     'myvar
     (lambda (sym val op buf)
       (when (eq op 'set)
         (push val (get sym 'prior-values)))))

There is no built-in automatic way; you will have to backup your old values any way you want; create a bunch of new symbols, put them into a hash table, a list, or whichever else strategy you prefer.

I think themes could be used for this.

Have a look at the approach taken by the logos.el package. There’s a section in the manual called Leverage logos-focus-mode-hook.

The idea there is that when you enable the logos-focus-mode, you’ll often want to change some other minor modes and variables too. But when you disable logos-focus-mode, you’ll want those other minor modes to revert to their previous value. The Logos package provides a few convenience functions to remember the previous values, and they are automatically restored when logos-focus-mode is turned off.

Perhaps the code for that could be made generic, instead of being coupled to the logos-focus-mode?