… that of Zcash (where your transaction is hidden amongst all outstanding transactions)
Not just all outstanding (JoinSplit) transactions: in Zcash, your notes are hidden amongst all other notes that have ever been created. Combined with the fact that coinbase must be entirely spent to a z-address using a JoinSplit, this means that the set of possible JoinSplit outputs that your JoinSplit could be using as an input increases continuously.