For example, at Level 73 in BDSP, Deoxys-N and Deoxys-A learn Hyper Beam, Deoxys-D learns Mirror Coat, and Deoxys-S learns Extreme Speed. Is it possible for 1 Deoxys to know all 3 of these moves? Via Move Reminder maybe?
When changing forms in BDSP, the moves it has will remain, and it can relearn moves from the move reminder.
Just tested this to verify.
Yes, they do stay.
I tried before and I first changed Deoxys to speed forme, got extremespeed, and then changed it to defensive form and got counter and mirror coat, and so I had a deoxys with extremespeed, counter, mirror coat, and psycho boost.