At the European Licensing trade show in October, Hasbro have announced that the 2019 Movie will "unveil an entirely new, exciting storyline for Transformers"... and doesn't note it being TF6 or TF7, or any number in the series.

This doesn't necessarily mean that the Bumblebee movie is the last one in the Bay-verse, as the 2019 could be the one that had once been noted as being an offshoot or CG movie.
Then again, the Bumblebee movie could make sense as a reboot movie, or a partial reboot like X-men and Star Trek, with the timeline being reset, allowing future movies start fresh without all the contradictions of the first five.

The link above has scans from the Hasbro brochure about Transformers, which has some other details about how the brand has been doing in Europe, and what licensed merchandise we can expect in 2018 (that could show up here as well).