Killing John Connor in the opening scene. Moreover, doing so merely so you can tell the exact same basic story and repeat the exact same Terminator formula all over again. Cameron had talked about updating the franchise but seemingly that meant nothing more than changing the gender of the savior of humanity and changing key names here and there. Hardly worthwhile and probably trying to appeal to the wrong crowd for a Terminator movie.
Furthermore, having John killed by a T-800 that looks like Uncle Bob is an incredibly cruel and bleak twist after the emotional ending of Terminator 2 and Sarah Connor's optimistic final narration about looking towards the future with a sense of hope. Cameron really pulled an Alien 3 here which was a bizarre choice from him - his disgust at Alien 3 is well documented for doing this very thing with the characters he created in Aliens.
There's also the issue of Carl and ageing Terminators generally that some have had a problem with. On the ageing front, I can kinda buy that the flesh will age but exactly in the way that real people's flesh ages? And in real people it's not just the flesh but the skeleton. Arnold's whole posture and the way he moves has probably changed in his older years whereas a solid Terminator Endoskeleton would not change in these regards.
Aside from that is the Carl storyline - which really requires you to....call into question...the whole ''it can't be bargained with, it can't be reasoned with - it doesn't feel pity, or remorse, or fear'' line from the first film. And according to Carl he had no further orders from Skynet beyond killing John Connor - how likely is that? Seems like it would be prudent of Skynet to have backup orders to safeguard or otherwise ensure its own existence and certainly not to allow a T-800 to become a blank slate that can be influenced by humanity to actually turn against Skynet of its own volition.
But well, that's what happened. Admittedly I found it a somewhat interesting premise and it generated a lot of great discussion in the All things Terminator thread.
The frustrating thing is that even if you're on board with it they go and kill Carl off at the end so that point of interest and the drama with Sarah Connor was going to be confined to this one film in their trilogy plan. Perhaps it could have been drawn out and expanded on more. It also seemed foolish to kill off Carl with 2 more films planned when it meant - presuming they would want further involvement from Arnie - having to come up with yet another really old T-800 model 101 and he'd probably be a good guy aswell because ever since T2 they just can't seem to resist going that way.
So for these reasons - combined with the aforementioned OTT action set pieces that feel cartoonish and a further step beyond plausibility from what happens in T1 and T2 - I file Dark Fate alongside T3, Salvation and Genisys as non-canon.