My 2018 did combined band work, heavy ball and strength program in the off season. He was shut down from throwing Nov 15- Jan 1 timeframe. Over the fall through Nov 15 he was throwing long toss 3-4x per week, then started the long toss back up in Feb when there was space to do so. He went from a 76 IF velo to 85 IF velo verified at an event. During HS and summer club season he somewhat maintained, then dropped to a 83 at a Aug showcase. Not much of a loss, but given the long season there was likely fatigue.
The plan is to do take the same approach with a IF velo goal of 88-90. With strength gains and added growth it's possible.
My 2018 was very committed to his arm care; activating his arm and surrounding muscle groups before stretching with team, and alway finding a way to get some long toss in when it didn't conflict with lots of throws in practices or games. He went through the past year with zero arm soreness.
So not much of a loss during season, and he worked out hard during club season to maintain strengths. He lost 5 lbs during season and somewhat maintained his IF velo.