I have the latest version of Pro and after playing around with the definition query and doing some googling; I concluded that Arcade is not supported in the layer's definition query expression. Please correct me if I'm wrong. If I'm right, this must be on the radar for a future version of Pro (I hope). Below is my definition query from ArcMap, and yes the expression works just fine in Pro. However, I have good reasons to move this over to Arcade.
CASE WHEN month(GetDate()) < 4 THEN
year(GetDate()) - 1