Did Israel follow pagan gods even though they had proof of the living God?
Yes. Even Israel, who had proof of the living God from their beginning and should have known better, followed after pagan gods. Jeremiah 44 records this, and it recalls Elijah's Mount Carmel controversy with the prophets of Baal, where Israel had been following them.
Les explains that verse 20's list—idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, and heresies—can be lumped together under idolatry or pagan worship. This bent to worship a false god has plagued the human race since at least the Tower of Babel and is innate in human nature. Even Israel, in Jeremiah 44, had proof of the living God from their beginning and should have known better, yet they followed pagan gods. That was the controversy Elijah had on Mount Carmel with the prophets of Baal; Israel had been following them.
But since we left off to burn incense to the queen of heaven, and to pour out drink offerings unto her, we have wanted all things, and have been consumed by the sword and by the famine.