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Legend
The DnD paladin comes from Charlemagne that is what it was designed from and that is what I am talking about when I say history. Are there other types of knight in history yes there are and DnD as often attempted to make them.
I am talking game history of DnD. Not gaming history in general.
And nothing is stopping you from playing it any way your group wants to play it. I am pretty sure the gaming police will not come to your house kick in the door and take your books if you choose to have paladins in your game that are not restricted to lawful good.
If 5E chose to do away with the alignment restriction it is not a deal breaker for me because I can house rule it the way I like it. The same way you can house rule it if they do it how you would prefer it.
D&D paladins have changed over the years. In previous editions, they had rules for non-LG paladins. It's an option people WANT. Give it to them. It doesn't hurt the Charlemagne concept if a paladin of freedom can also heal the sick.
The character builder could actually stop people. Living Whatever could stop people. DMs who are afraid to venture from the rules could stop people. You have to keep these factors in mind. It's not just fluff anymore when parts of the game hard code it away, and when there is a known history of DMs who have no confidence in themselves.