readyToStart was used in waitForEpochAck to check that the leader passed a certain point in the code (right before getEpochToPropose), and this check was not sufficient because it doesn't mean that the leader reached the getEpochToPropose or the waitForEpochAck barrier. ZK-1194 fixed this by explicitly checking that the leader reached the right barrier, so it no longer checks readyToStart.
The only place where readyToStart is still used is in Zab1_0Test.java, where it is read to check that Leader.lead() passed cnxAcceptor.start(). As far as I understand, checking that leader.cnxAcceptor.isAlive() achieves the same (it might actually be better if cnxAcceptor.start() doesn't atomically make cnxAcceptor alive).
Happy New Year!