And what if your player insists on playing the 2014 paladin?
I don't say that as in a gotchya. I say that because I can already see the bot filled threads on Reddit having this exact question: "My player insists that WotC said you could mix and match, so now he is playing the broken 2014 paladin with a touch of warlock, and it's throwing encounters way off. What do I do?"
And then we'll see a bunch of people mention session zero and to communicate. Then the DM will respond with, "I didn't think it would matter, as WotC said we could mix and match. So it's only become a problem now that they are level 8."