I'm pretty new, but according to the official documentation on test networks, Alphanet is deprecated and got replaced first by Babylonnet and now by Carthagenet.
Yet there is an alphanet-tag, but no carthagenet-tag.
Babylonnet was a test network which ran the Babylon protocol. It was spawned after the injection of the proposal for Babylon. It ended its life on March 31st, 2020 as Carthage replaced Babylon on Mainnet on March 5th, 2020.
Alphanet was the test network before Babylonnet. At the end of its life, it was running the Athens protocol. Bootstrap nodes were shut down after the Babylon protocol was activated on Mainnet.