2019 mMac Pro High Sierra AMD GPUS's EFI boot screens ?

Hard to say until it's released. Given the increasing lack of traceability of their decisions, be prepared for anything.

 

The only thing we know about the future version of the Mac Pro, is that it will be expensive.

Beyond that, we can assume that it’s likely Apple will govern expansion options by using proprietary parts. Expandable as long as you use custom Apple variations of an otherwise generically available component part.

 

^^^^^^^^^^ And I don't really mind that, provided the do offer all varieties of desired expansion with modern tech.