Are there any bow executions? I havent seen one so far

I don't think so; I guess it would be possible with the bow bash, but I'm not sure if it would initiate a finishing move.

As someone who has been playing 60+ hours and has killed nearly everything with a bow - no. I can say definitively that bow executions do not exist. Which I find strange, but anyway . . . .

I think there should be atleast one when an enemy goes down to his knees with 10 health left. Even a slow motion shot would be cool.

But how would u like them to do it ? They already fall over... and it's not like they could blow them up in pieces like in fallout
Dont get me wrong, i also would had liked bow executions but then i started to think about it, arrows are already sticking out in bodies wich is cool especially when it's in their eyesockets, the only possible thing that they could add is a slowmo arrow camera vieuw on the 'kill shot' but that would be hard to do since there is no VATS like in fallout so the calculation of impact happens 'realtime' i guess.