I think that's the year when the "Poker, Chess, and the AD&D System" article came out in Dragon Magazine, where Gygax said:
Notice that TSR owns the ADVANCED DUNGEONS & DRAGONS game system. Well, they own the DUNGEONS & DRAGONS game, too — just as Parker Brothers owns MONOPOLY and THE MAD MAGAZINE game. TSR holds the copyrights to the D&D and AD&D games. They own the Trade Marks. Use of either must be by TSR or with its permission. Neither game is public domain. No other firm can make any commercial use whatsoever of either game without permission from TSR.
And later in the same article:
The AD&D game system does not allow the injection of extraneous material. That is clearly stated in the rule books. It is thus a simple matter: Either one plays the AD&D game, or one plays something else, just as one either plays poker according to Hoyle, or one plays (Western) chess by tournament rules, or one does not. Since the game is the sole property of TSR and its designer, what is official and what is not has meaning if one plays the game.
The other FRPG companies, who sold supplements that were "compatible with Dungeons & Dragons", or "for use with fantasy role-playing games" thought that Gygax was trying to put them out of business.
Most gamers just scoffed at the article, though, and continued playing the game the way they wanted to.