The conditions for the bounty are:
The bounty fund will pay out as funds are available according to the signers best judgment for completed work proposed in this thread that furthers the goal of making improved transaction privacy a practical reality for Bitcoin users.
And, having considered this for a while, my perspective is that no-one has really achieved this.
Coinjoin (as currently implemented) has a problem: coinjoins with a large number of participants and also similar/equal output amounts are easily identifiable on the (public) blockchain. This simply reduces fungiblity in a different way: now, outputs from mass coinjoins can be identified as "coinjoin related" and labelled as such.
Payjoin and PaySwap (link:
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-April/016888.html) are the real solution, and they are as of yet unimplemented by anyone.
Coinjoins
must look exactly like any other transaction on the blockchain in order to make Bitcoin transactions truly private.