You can use the favicon cache as storage too, by redirecting users across domains. It's been proposed as a potential fingerprinting risk[0], and if a browser naively reuses the cache for incognito mode, it could be used to track users across browser profiles.
My thoughts instinctively went to "this has to be being used for fingerprinting" when I read OPs blog. Are anti fingerprinting measures taking into account the use of the canvas api with favicons?
The link to the supercookie site is dead unfortunately.
[0]: https://www.schneier.com/blog/archives/2021/02/browser-track...