Anything which comes along which does not contradict existing canon in terms of content, continuity, and consistency is canon.
Official films and television trump all else.
I do not consider parts of TOS to be canon anymore, because they have been contradicted and rendered inconsistent by later films and shows. The events of the TOS episodes happened, but not in the way presented. This can likewise be said even about some TNG episodes.
As Trek has evolved it has changed its terminology, use of technology, and a great deal many other things.
One thing people seem to forget about Trek is how important time travel is to the storyline. It happens to some degree in every series at least once per season. To think that it only happens to the ships we see on film is asinine. Every Federation ship ever sent out likely experiences it at least once in its life (likely more often), compound that by the vast number of warp capable factions in the universe.
This means alternate timelines are springing up all the time, and it is fine if one episode contradicts another.
However it is important to have a concise steady line of events connecting everything. That is where canon lies. It isn't in individual events of books and movies, but in certain key aspects of Trek.
There is two forms of canon for me, in this regard ;
Concise Plot ; If an episode or piece of fiction presents something which disrupts an ongoing plotline, then clearly it must be part of an external timeline. STO is part of a concise plot, and thus unless a new Prime Universe show or move comes out that contradicts that plot it is canon.
Consistently Trek ; There are certain aspects of Star Trek that must be adhered to, and going outside of them makes things bizarre. Part of why I have trouble with Star Trek V for instance is how quickly they get to the center of the galaxy. That is inconsistent as hell with Trek overall. While the events of STV are definitely canon, I feel the story is a largely embellished one. Either that or key parts of the plot are missed in the telling we witnessed. There would have to be a wormhole or something.
STO is largely consistent, with embellishment to make it a fun game. I can see the plot of the missions as being largely canon by my first criteria, while many of the finer details fail my second criteria.
Which is fine, because these lesser issues are largely part of our individual experience in the game and have nothing to do with the plot. I do not think everyone having Quantum Slipstream Drive is canon or that Feds are flying around with disruptors and antiproton weapons on their ships in droves in the Prime universe, but it is fair to say that Klingons being at war with the Federation is canon until a movie or show contradicts this. At which point, STO becomes an alternate timeline.
I am grateful for the threads that link JJverse, (including Countdown and obviously Spock Prime), with everything that came before, because they could have not done anything at all to bridge realities.
I don't like, however, the notion of endless copies of characters, (or real people for that matter), in an infinite number of realities. It cheapens the uniqueness of an individuals life-soul.
If that really bothers you, then I am unsure how you can be a fan of Trek at all.
This is a universe where both Spock and Data can copy themselves into new bodies and be considered the same people. These weren't even personality transfers seeing as the originals were still walking around afterwards long enough to sacrifice themselves, these were copies. Data may be an android, but Spock basically copied his soul.
One might say that the Soul is broader than just one being and is what ties all our different selves in different realities together. To me that is actually pretty awesome and does the opposite of cheapening us.
Or these infinite selves are still unique individuals. We just start with similar source material. I think that if this is true, our uniqueness actually matters a lot more. Being born an identical twin doesn't cheapen someone's individuality.