D&D Rules Cyclopedia (1991), Ch. 10, page 127:
"In the game, there are five normal ways for player charactes to acquire experience:
1. By Role-Playing Well
2. By Achieving Party Goals
3. By Defeating Monsters and Opponents
4. By Acquiring Treasure
5. By Performing Exceptional Actions
...Exceptional Role-Playing: Finally, whenever a player performs some role-playing that really impresses the DM, the DM can award him some extra experience. Such a demonstration of role-playing might be an emotional encounter with an NPC, reacting to the loss of a loved one, spontaneously composing a speech that motivates the army of NPCs and improves their mood, and so forth. If the demonstration of role-playing enlivens the game, impresses the DM, or makes for a very memorable scene, the DM may decide to give the player a bonus. The bonus given to the character should be about one twentieth the points it takes him to get from his current level to the nest. Don't use his current experience point total: use the base number... A character should not receive more than one such bonus in a single play session. even if he role-plays well throughout the session."
There are many more examples in the various rules for granting XP for role-playing. The point is that such has indeed been codified and part of the game for a very long time.