Albert Barnes on John 3:24

For John was not yet cast into prison. See Lk 3:20. The mention of this shows that John was not imprisoned till some time after our Lord entered on his ministry. The design of John was to call men to repentance, and to prepare them for the Messiah, and this he continued to do after our Saviour commenced his work. It shows that a minister of religion should be industrious to the day of his death. John still toiled in his work not the less because the Messiah had come. So ministers should not labour less when Christ appears by his Spirit, and takes the work into his own hands, and turns many to himself.

(x) "For John was" Mt 14:3