Comment by ergl

You're right, I misspoke in my original comment. See https://news.ycombinator.com/item?id=37560124

What usually happens is that a leader won't confirm an operation as successful until such operation has been applied in a quorum of replicas (see: synchronous replication).

In theory, nothing prevents a leader from accepting new writes even if it can't reach a quorum, provided it never allows reading operations that haven't been replicated to a number of replicas.