Historical Rules · 1 May 2013
Rule 605.2
Official source text from the 1 May 2013 Rules release
A mana ability remains a mana ability even if the game state doesn't allow it to produce mana. Example: A permanent has an ability that reads "{T}: Add {G} to your mana pool for each creature you control." This is still a mana ability even if you control no creatures or if the permanent is already tapped.