Alice has Zcash that was sent to her by her chosen exchange
Alice sends Zcash to Bob via shielded transaction
Bob sends Betty Zcash also via a shielded transaction
Betty wants to cash out and sends the money to an different (or same) exchange via a t-transaction
Can Betty’s exchange figure out that Alice or the first exchange ever held these coins?
So you want to sell drugs and is wondering if anyone can find out?
Start here: Privacy Recommendations and Best Practices - Zcash and there are also some decent blog posts i.e. here A Shielded Ecosystem - Electric Coin Company and Anatomy of A Zcash Transaction - Electric Coin Company
Simply put the shielded transaction information wouldn’t be visible on the blockchain but depending on the timing or exact amounts used it may be possible to find some linkages i.e. if the exact same amount (minus the standard transaction fee) was used for both t-transactions that occurred within a short space of time.
This page appears to answer my question:
Unshielding a transaction by sending from z-addr to t-addr does not reveal any previous history.
I guess this is why the trusted setup forgery problem exists. If you were able to trace back to mining then forgeries would not be possible.
They are actually unrelated. Unlinkability is ensured by the very nature of zero-knowledge proofs - the linking data simply isn’t there (modulo any end-to-end linkability due to matching amounts etc.). The trusted setup forgery problem is a side-effect of using an efficient ZKP - there exist ZKP systems that don’t have trusted setups, but their proofs are either too large or too slow to use in a cryptocurrency (probably both).