For the first time ever, the producers/actors/HBO can keep the fate of a character in the dark for everyone, since there are no more books available. It's obvious they had to think carefully about what they should say about Jon's future (Kit does a lot of interviews after all, he's one of the big 3), and saying "I don't know if I'm back, just wait and see" would lead to never ending questions about his future. Saying that he's done they will leave him alone, he won't be part of season 6 (except for a couple of quick shots done in secret at the very end of the season) and they can fool the audience.
The show has done a much better job than the books creating the impression that Jon might be gone forever. In the books, thanks to all the fake-deaths, apparent deaths and resurrections, nobody believed for a second Jon was gone forever. The show has everybody freaking out about the very real possibility.
I'm absolutely sure they have NOT filmed any scenes for season 6. Season 6 was not written or designed yet.
The smart thing keep Jon's fate a mystery would be to resurrect Jon at the very end of the season, so they would only need Kit Harington for a couple of days of interior shooting; remember, we hardly get any news of what they shoot in the studio (or in Ireland, for that matter). Only things shot in the southern locations leak frequently.
Also, that would make more dramatic sense. Having Jon back at the beginning of the next season would be just a cheak plot-device to get him free of his oath. However, the return of the saviour after a very bleak season 6 would be a powerful way to set up season 7.