As someone who have been involved with chess engine community for the last 15 years or so (not anymore) I can tell you that, until recently, almost every single

As someone who have been involved with chess engine community for the last 15 years or so (not anymore) I can tell you that, until recently, almost every single new engine, that has been written from scratch instead of copying an OS implementation, almost always left out en passant and castling in its first few releases. Even when they did implement those rules in to the engine, they did so grudgingly or as an after thought. I wonder why. Now, everyone just copies the few dozens of OS options available out there and use their move generator which support all the rules, they just tweak with the piece values and algorithm to give a "personal touch" to the engine.