This is one of the reasons why the Expires header is used (for libraries on CDNs, not tracking scripts/images): the CDN knows that you downloaded the library, with one referer per year, the browser doesn't contact them if you visit another page using the same library. If enough sites that you use link to their copy of the library (same version, same library), then this isn't a very useful fact about you.
I think the worstcase latency/reliability is a bigger issue with such CDNs.
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