I wouldn't be holding your breath for one of these for the Mac Pro. Remember - Apple will have to design a board for the chip because Apple went with MPX connector.
The MPX bay design accepts regular GPU add in cards just fine. It also has a two 8 pin molex connections which can power 300W just fine too.
There is about nothing that Apple needs to build custom here to work on graphics driver software at all.
A down-clocked 6800 would probably work more than reasonably well in a speed bumped iMac Pro. (speed bump the Thunderbolt v3 controllers and would pair nices with the XDR monitors ) . There is no MPX hurdles to jump there either.
There isn't a humongus leap either from the baseline W5700X design either. GPUs packages are a different size but the power delivery/conditioning is a bit different . 16GB of VRAM. in both cases. 256-bit bus in both cases ... so layout changes minimal.
The aspect of the MPX module directly associated with the MPX connectro don't have to change at all if just keep the same two Thunderbolt v3 controllers on the modules. The PCI-e and USB feeds are the same. The DisplayPort v1.4 outflow is the same too. In that context there is abosolutely nothing 'new' to design there. ( might be some minor trace reflow to do but it almost exactly "same stuff , different day".
The large time sink here is likely doing the software drivers and that can be largely be driven with a reference card 'test mule'.
The big blockers for Apple are far less technical. First, motivation to do work. ( if going into another Mac then more motivation. If the whole Navi 21 family is just Mac Pro then 'kick to can' ). Second , Price. AMD is going to be able to sell every last drop of any working die here. Nvidia is blocked on supply (rampant product shortages ) . AMD has gobs of other 7nm parts to get out. Apple probaly wants some Scrooge McDuck discount price that AMD will just chuckle at for more than several months. Pretty likely Apple is at the end of the line for supply demand also.
( and the 6900 in particular is likely the binned , of the binned , of the binned dies. Kind of doubt that will make a MPX module just on pricing just won't fall much. )
I'll be picking up a 6800 for my PC on Black Friday. 😁
If they are around. AMD's supply may be better than Nvidia's for the next 4-5 weeks but doubtful there is going to be gobs of excess. AMD sent out emails/notifications to suppliers to restrict to one-per-customer to avoid shortages.
AMD has pushed the 6900 back to probably free up some supply, but it isn't like nobody wants these so the demand will be initally low.
P.S. if the Cyptocurrency code and/or data has high hit rate with the "Inifinity cache" that will boost short term demand also. ( at least until AMD has a more computational focused card to sell them. )