It can be argued that the new skill trees are not considered gameplay mechanic since it is not part of the base game. If you can continue to play without it then it's just extra DLC, which companies can choose at their discretion to charge.
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Do you understand what gameplay mechanic is? mechanics have nothing to do with "BASE GAME" mechanics are how the game is played and is not tied to how the game started, you can add mechanics or remove them if you want with an update of the game.