The new Terminator movie Terminator: Dark Fate, due for release November 1st, 2019 is reported to disregard the events depicted in Terminator 3: Rise of the Machines and Terminator Salvation from the continuity of the Terminator movies. This is similar in concept to the new Halloween movie which released October 19th, 2018, which disregarded all sequels and reboots of the franchise and was a direct sequel to 1978's Halloween. Although Terminator franchise creator James Cameron is involved with the movie as a producer, and creative consultant, with a writing credit according to The Terminator Fans, the movie is being directed by Deadpool director Tim Miller.
Published September, 27th 2017 the above "conversation" with Cameron and Miller, hosted by The Hollywood Reporter's Matthew Belloni, gives an insight into the thinking, writing, and development that has gone into the new Terminator movie. On the subject of how the movie will exist in the already established timeline, Miller outright refutes that they will use alternate (reality) timelines, which together with the humorous inference in Cameron's earlier comment that Terminator 3: Rise of the Machines and Terminator Salvation may exist within an alternate timeline suggests that the new Terminator movie may have another in-universe explanation as to how its timeline, which differs from that of the two aforementioned sequels, can exist.
The Future Is Not Set?
As evidenced by its predestination paradox narrative, The Terminator was originally conceived as a closed story. This and his deal to make Aliens for 20th Century Fox is likely why Cameron sold the production rights to The Terminator so frivolously and for such a low price (for $1 to producer Gale Anne Hurd). However, when the possibility to return to the franchise arose in 1989 Cameron, reunited with William Wisher Jr. and conceived of an extension to the story explored in The Terminator.
The original intention of Terminator 2: Judgment Day, as exemplified in the deleted Future Coda scene (pictured above), was to explore a more optimistic narrative than that depicted in The Terminator, whereby the future war was to never happen. As explained in Time Travel Explained, had this ending been used this would have resulted in a grandfather paradox of impossible circumstance - How can John Connor exist if Kyle Reese was never sent back in time to father him. Understandably, such an impossibility unravels the entire narrative of the franchise. Unfortunately, with Cameron's involvement in the new Terminator movie, it is likely this scenario may be revisited as the conclusion of the Terminator franchise. While Cameron is free to create whatever future he wants for the franchise he created, one fact remains - the story of the original movie, The Terminator, was about a predetermined fate, an inescapable causal loop of events.
Although events subsequent to The Terminator became somewhat convoluted, Terminator 3: Rise of the Machines did reinstate the predestination paradox narrative of the original movie by revealing that Judgment Day had not been stopped in Terminator 2: Judgment Day but merely delayed. Terminator Salvation added to this showing that a delay of Judgment Day had unforeseen repercussions, as shown with Skynet developing the series 800 "Terminator" 10 years earlier than Kyle Reese had stated when he was being questioned by Dr. Peter Silberman in 1984 in The Terminator.
While casual fans may not have enjoyed the aesthetic of Terminator 3: Rise of the Machines and Terminator Salvation, it can be argued that, on a narrative level at least, both of these movies addressed the issue presented by Terminator 2: Judgment Day that could have damaged the franchise irreparably, and as such, they should remain within the franchise.