After reading this answer, where Peter Wuille talks about how a node can't really tell if it is synced or not, it got me wondering if there is a way we can tell that a node is as synced as it possibly can be given the peers it is connected to.
Could we add an RPC call like
getsyncstatus that would have the node ask its peers for their highest known blocks and if a majority (or some percentage) of their peers all agree on the best known block height, then it could use that info to be able to say (with not perfect reliability) how synced the node is?
So, for example, if a node has 8 peers, and 7 of them are returning
330448 as the best block height, and the client has information about block
330448, then it could give a status code and message signalling that it is up to date. If the peers give different answers (which could happen if you connect to a malicious node that is falsely reporting it's best known height OR if other peers are not fully synced), then it could give a status code and a message indicating that the search was inconclusive and they should try again later. And lastly, if 87.5% of nodes are saying
330448 and the node is only synced up to block
1000, then it can give a status code and message indicating that the node is likely not sufficiently synchronized.
I think this would mainly be useful as a way to determine whether servers are sufficiently synchronized with the network for use on some application. The responses to this question describes a way to see if nodes are up to date, but they all seem kind of hacky, and I'm not sure they would be reliable enough.