PayJoin is a privacy improvement for bitcoin. In the case where a customer pays a merchant, they both together co-operate to create a single bitcoin transaction which mixes both their coins and masks the payment amount.
Transaction surveillance companies heavily depend on the Common-input-ownership heuristic which is broken by PayJoin transactions. So if those transactions became even a little bit widespread they could massively decrease the reliability of blockchain surveillance. Merchants and customers who adopt PayJoin would find their privacy improved from anyone analyzing the blockchain, for example a surveillance company spy would find it much harder to figure out which addresses and transactions belonged to a particular merchant that was using PayJoin.
PayJoin transactions are indistinguishable from regular bitcoin transactions by design, so it's very hard to get an accurate number for how common they are.
The PayJoin protocol standard most likely to get adoption is BIP 0078.
|??||Maybe / Haven't checked / placeholder|
|Planned||The developers said they plan to|
|Non-BIP78||Implements a form of PayJoin but not BIP78|
|PR Merged||In the case of software, code has been written and merged, and it will be in next release.|
|Yes||Feature has been released|
|Bitcoin Wallet for Android||No||No|
|BTCPay||Yes||First implementer of BIP78 payjoin for merchants.|
BTCPay stores which accept PayJoin
P2P exchanges make the most sense as early adoptors of PayJoin. All exchanges are welcome on this list of course.
Bitcoin casinos are very natural early-adopters of PayJoin. An early protocol specification for it, called bustapay, was created by the owner of a bitcoin casino.