in 2019, WIFI 6 it's still in final steps, I can see Apple making test for at least June 2020 before introducing it in any Mac, together with some extra upgrades, Macbook Pro 16" could be the first, adding new form factor and some little changes, iMac could be next, if there is something more to add (maybe an intel updated chips…) as it would be very very odd from Apple to update just one component in their products.
Mac Pros could be the last one, as they work with huge loads of data and wired connection is expected.
the bigger you are, the louder the mistake you make is, Apple is the king here, a lot of cover with new products, the same for the complaints, so they need to be very careful with that.
The retina macbook 2012 which I have, was almost 2 years in the market with a extremely annoying WIFI-bluetooth problem, they took a lot of time to fix it and was painful. Luckily was a firmware issue and could be solved remotely (but 2 years!!!).