In Backdrop, regions are all now determined by layout module. Therefore region-specific template suggestions for blocks (which allowed the ability to do, for example, block--header.tpl.php) has been removed.

Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Module developers
Theme developers
Related Github Issues: 
https://github.com/backdrop/backdrop/pull/309