As far as I know it has not been officially commissioned, but there is every expectation that there will be a series 6. The return of Shrimpie would seem to be setting up the exit of Rose (Lily James) due to her commitments to Cinderella. This would not be necessary if the series were to end after this year.
Series 6 has now been officially renewed, but there is speculation that it will be the last season.
But there are also counter-rumors that Downton Abbey's U.S. producer "NBC UNIVERSAL" are appalled that such a hugely successful show could be 'axed' whilst still so popular, and are putting pressure on Julian Fellowes to hand over the writing (of Season 7 and beyond) to their own team of scriptwriters.
This is what I hate about rumors - they always conflict and you never know what to believe.