Same tracking is possible by embedding data in last-modified/if-modified-since, or location of cacheable redirects. There are also other variations that just u

Same tracking is possible by embedding data in last-modified/if-modified-since, or location of cacheable redirects. There are also other variations that just use resource bodies, e.g. cache HTML that opens an iframe with a unique URL, JS that contains constants storing data, CSS that opens user-specific URLs via imports, etc. For tracking you just need any unique identifier, so (lack of) flexibility doesn't do much. You can even track users with HSTS cache that caches only 1 bit per domain.