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.
GasbYear LIKE
CASE WHEN month(GetDate()) < 4 THEN
year(GetDate()) - 1
ELSE
year(GetDate())
END
I agree with Dan - I quickly checked and don't see this submitted on the Ideas site. I would recommend that you call technical support and provide the use case that can be captured as an enhancement request for ArcGIS Pro. That is a more direct route into the system, but like Dan mentioned, it is also good to post the Idea as a way for us to gauge the need for this across the community. Thanks.