I don’t think I’ve ever seen EX explicitly proposed by any English game for me to use.
X came from the Infocom games. Now, there are/were English games that don’t support X, but some older games supported abbreviating any command to the shortest number of letters it would take to distinguish that command from all other commands in that particular game. Their parsers were programmed quite differently and RAM was a consideration. Such games would also accept any abbreviation inbetween that short version and the complete word. In such games, E was inevitably east, and then the shortest thing you could usually type to distinguish EXAMINE from E was EX. That’s really the only English context in which I’ve seen it come up, and it still wasn’t advanced by the game; it was something the player would hit on when they realised they wanted to type less or could type less. So a game like that would tend to also accept EX, EXA, EXAM, EXAMI, EXAMIN etc. automatically.
Short answer: I don’t think EX is common and was mostly a symptom of the way some older parsing systems were programmed when it was used.