According to Hasbro's investor call at the NYC Toyfare, there will be a hard reset of the current Transformers series, and the intended sixth film has been cancelled following the poor performance of Last Knight. No TF movie past Bumblebee is slated for the next three years
http://news.tfw2005.com/2018/02/16/tfw2005-coverage-toy-fair-2018-hasbro-investor-preview-event-358628
http://news.tfw2005.com/2018/02/16/tfw2005-coverage-toy-fair-2018-hasbro-investor-preview-event-358628