Deprecate backend parameters

[piccolbo]

I've seen too many incorrect uses and it was an abstraction-piercing feature to start with. It's time to kill it before we have a code base of non-scalable, non backend independent code and we are stuck. If you rely on this feature, please describe your use case in the comments and we'll try to support it in a different, higher level way.

[piccolbo]

Slipped through the cracks, moving to the next milestone