How do I add a custom block to a view? In preset views like the Taxonomy Term one (/taxonomy/term/%), it can be done by adding a custom block to the footer, but it is not controllable as block layout on /admin/structure/block.

Is it possible to add a region to a custom view template?
Simply go to /admin/structure/block and add the block to a region.

In configure block, under Visibility, click on pages tab and type path with wildcard /taxonomy/term/*. 

Therefore, this block will only be visible in your taxonomy page view.