Is 2 John 1:10-11 about people of different religions, or about groups like Protestants and Mormons?

You have to see the big issue in John's epistles, which is the Antichrist named in verse 7: the many deceivers who do not acknowledge Jesus Christ as coming in the flesh. This grew out of a heretic named Cerinthus, an excommunicated man who went around teaching that Jesus was not God, only like God; he was a forerunner of the Arian crisis that came a century or two later. That is why John keeps repeating that if someone denies Jesus is from God or is God, do not talk to him, do not receive him into your house, have no association with him. So it is a specific kind of heresy John targets, the denial of Christ's divinity and incarnation. But as a Catholic you can apply the principle to many similar cases. When a Mormon knocks on your door and offers you another testament of Jesus, or a Jehovah's Witness comes, both of whom deny that Jesus is God, or anyone who denies the Trinity, you can tell them plainly: you hold a false doctrine that Jesus is not God, so I am sorry, you cannot come into my house, and I do not want to talk about it. That is the answer you can give to every Jehovah's Witness, Mormon, and the like.