I am also having issues with this as I cannot find it in filters in Revit 2021. Rather than overriding each filled region and remembering the exact percentage, it would be great to make a filled region by default xxx opaque no matter the view template. The issue is obviously that you have to add this on each single view template.
(Offtopic) As we are talking about filled regions it would be great if the whole process of it is reworked as I find it the least successful part of Revit. 1. There should be be a setting to automatically change all the lines for all selected filled region all at once (rather than going into it) and 2. There must become a smarter way of selecting regions and lines to make sure it is perfectly enclosed. The amount of hours I have spent trying to find some overlapping line or this tiny gap of 0.1 mm that somehow was not spotted in the "show" part.