Is it appropriate for me to send a farewell message on WeChat and then shut down my boyfriend for three days?

It's definitely not appropriate. First of all, if you say "goodbye", people will feel where you are going and where you really want to leave, and friends will be curious about you. Then you keep your phone off for three days, big deal. Your friends will think you are leaving this world, and everyone will worry about you because of your behavior, and then look for you everywhere. If nothing happens, that's fine, but if something happens, it's in big trouble. On the other hand, if your friends find out that you are playing with them, they may look down on you in the end.