In Harry Potter and the Methods of Rationality by Eliezer Yudkowsky a concept called the 'Interdict of Merlin' appears: (all emphasis added)
His hand on the doorknob, Harry Potter already inside and waiting, wearing his cowled cloak.
"The ancient first-year spells," Harry Potter said. "What did you find?"
"They're no more powerful than the spells we use now."
Harry Potter's fist struck a desk, hard. "Damn it. All right. My own experiment was a failure, Draco. There's something called the Interdict of Merlin -"
Draco hit himself on the forehead, realizing.
"- which stops anyone from getting knowledge of powerful spells out of books, even if you find and read a powerful wizard's notes they won't make sense to you, it has to go from one living mind to another. I couldn't find any powerful spells that we had the instructions for but couldn't cast. But if you can't get them out of old books, why would anyone bother passing them on by word of mouth after they stopped working? Did you get the data on the Squib couples?"
Harry had asked Hermione about that earlier - on the train to Hogwarts, after hearing Draco say it - and so far as she knew, nothing more was known than the word itself.
It might have been pure legend. But it was also plausible enough that a civilization of magic-users, especially one from before the Interdict of Merlin, would have managed to blow itself up.
The line of reasoning continued: Atlantis had been an isolated civilization that had somehow brought into being the Source of Magic, and told it to serve only people with the Atlantean genetic marker, the blood of Atlantis.
and Chapter 80:
This is the Hall of the Wizengamot; there are older places, but they are hidden. Legend holds that the walls of dark stone were conjured, created, willed into existence by Merlin, when he gathered the most powerful wizards left in the world and awed them into accepting him as their chief. And when (the legend continues) the Seers continued to foretell that not enough had yet been done to prevent the end of the world and its magic, then (the story goes) Merlin sacrificed his life, and his wizardry, and his time, to lay in force the Interdict of Merlin. It was not an act without cost, for a place like this one could not be raised again by any power still known to wizardkind. Nor yet destroyed, for those walls of dark stone would pass unharmed, and perhaps unwarmed, through the heart of a nuclear explosion. It is a pity that nobody knows how to make them anymore.
Is there any mention at all of the Interdict of Merlin in Harry Potter canon (anything written, spoken of or referred to by J. K. Rowling), or was this simply a plot device added by Eliezer Yudkowsky?