My guess would be that the headphone jack will stay as I see no urgent need for Apple to remove it.
Unlike on the iPhones, the internal space gain is rather negligible, and it wouldn‘t allow Apple to put more speakers on the iPad like it did with the iPhone. Water resistance would be the only reason I could think of for Apple to remove it, but I doubt Apple is in a hurry to get that feature onto iPads – after all, how often do you pull out your iPad while walking down a rainy road, or get it into the close proximity of a pool/watery area. On the iPhone, the removal of the jack allowed for all these other features but on the iPad it might just not be a necessity in order to move forward.
Something else to consider is that Apple wants the iPad to be a laptop replacement whenever possible, and they kept the headphone jack on the MBs and redesigned MBPs as the only port besides USB-C, so if you consider the iPad to be closer to the MacBooks than to the iPhone then it would make sense for the jack to stay aswell. And that‘s the direction where Apple is heavily pushing the iPad Pro both software- and hardware-wise for the last few years.
That said, the 2018 iPad Pros are rumored to be the possibly largest iPad update to date with a big design update, Face ID, no (or reduced) bezels and an iPhone X-ish gesture control scheme. If Apple does want to remove the headphone jack from the iPads anytime soon then this year would be the best opportunity to do it, they could easily excuse it through the new design and would have plenty of new shiny features to make up for it, more so than the last couple of years where the design remained mostly unchanged and where the port removal would have been harder to justify.