What does "unbroadcast" mean? What does it mean for a transaction to be "successfully" broadcast?
Unbroadcast (introduced in PR 18038) is defined as when the initial broadcast of a transaction hasn't been deemed successful. Every 10-15 minutes a node re-attempts broadcast of unbroadcast transactions.
An initial broadcast is deemed successful by the mempool if it receives a single
GETDATA request for that transaction (ie a peer requests the data for that transaction). The mempool keeps track of transactions that it initially broadcasts and maintains a set of unbroadcast transactions containing the txids. It doesn't necessarily know the reason for a "failure". It may be that the transaction was dropped and didn't reach peers or it may be the case that peers weren't interested in that transaction for some reason. Peers are unlikely to already know about the transaction as it was originated by the source of the initial broadcast.