not if the link itself is unique for each user trivial to implement, this is how email campaigns track what you clicked in a email ( email after all do not server js)

doesn't that only work for sites the link creator has control of? i don't see how the technique can be used with links a search engine finds and then gives out to anon users.

This is how it would work.

1. The search engine adds itself to the tracker list in the magnet URI (&tr= query parameters), but with a unique subdomain and using HTTP (i.e. it adds http://asdfkja.skytorrent.in/announce to the tracker list)

2. The client announces itself to the tracker, sending the unique "Host" in the GET request (asdfkja.skytorrent.in in the example).

In that case the plain text link would be just as trackable.

magnet links are user-side. They're like email addresses or phone numbers.

This doesn't prevent their users from being tracked.

Email addresses can be used for tracking by having each request for an address (e.g. each GET of a "contact us" page) return a different email address (e.g. contact43327@example.org). Checking which address gets used allows the GET request (and all associated information) to be associated with the email message (and all associated information).

Likewise, magnet links can be tracked by looking for torrent clients trying to access them. A sibling has pointed out that unique tracker URLs can be used for this. Another way would be to make up a unique content hash for each request, then lurk on the DHT looking for queries for those hashes.

If the operator has no qualms with transferring data (e.g. being directly exposed to copyright infringement) they could even service such requests, with the user being unaware of the fact they're being tracked: the operator alters the hash by making some innocuous "watermark" change to the content, e.g. altering a filename inside an archive; each time a chunk is requested, the operator fetches it from the "original" and passes it on.