Star Wars Blog: “We know you have all been eagerly awaiting to hear when we plan to release our Onslaught expansion. The feedback from our Public Test Server (PTS), along with the data from our internal play tests showed us we need more time. We havent released all the changes to PTS, and we want time to make adjustments based on your insight. It was a difficult decision but with our commitment to delivering you a quality experience, weve decided to push the release date for our new expansion to October 22, 2019.”
Source: N4G PC Star Wars: The Old Republic Onslaught Expansion releases October 22, 2019