No mention whatsoever about any hint of Harry leaving
He is DEFINITELY leaving as there's been a BBC press release about it.
The news article that quoted that release then says that he leaves after making a go of things with Nikki, they fail, he gets a job in America...Where that info comes from I don't know. Maybe the press release too.
But they said this was his last ep. So either that's the last we'll see of him (which is a ridiculous ending for his character tbh) or he'll be only in the first ep of the new series.
EDIT: According the the fan site, Tom decided to leave AFTER production was finished on S15 and so they couldn't do a proper exit for Harry (ffs man would it have killed you to come back for one ep!)