I mean... I don't think that objectively is the right word here. I think that... unfortunately, the relevance of a specific subplot can't be objectively described. I mean, when does a plot become relevant? X number of cutscenes, X number of mentions, X number of name drops?
It's relevant when, if you cut it out, the game doesn't make sense. If you cut out that plot, then Vexen nevers joins the Organization because he lacks the desire for atonement. If Vexen doesn't join the Organization, then it's not complete. If it's not complete, then there's no 13 vs 7 clash. If there's no clash, there's no KH3.
I think the point that's trying to be made is that... in terms of gameplay and story, we as the player hardly ever make any steps towards reviving Roxas.
That's more your personal problem than the game's problem, honestly.
Sora recognizes, occasionally, that people are working towards this behind the scenes. But for the vast majority of the game (AKA the Disney Worlds) Roxas is an afterthought. As soon as Sora touches down in say... Toy Box, all mentions of Roxas, the urgency of bringing him back, it all goes away. He ceases to be relevant to the story. In KH2... this wasn't really the case. When Sora lands in Land of Dragons, he talks about finding Riku, stopping the heartless. It's tied to the main storyline. In KH1, there's a super easy way to continue plot relevance with the Council of Villains, the search for Kairi, Riku's continued appearances, and so on.
So? "Save Roxas" is not the plot of KH3, the plot of KH3 is "Defeat MX" (more or less). The overarching post-KH2 plot is "Everyone connected to Sora need to be saved" which was first set up by Blank Points, and that means Terra, Aqua, Ven, Roxas, Naminé and Xion. They know where Aqua is (and they know that they need her to find Ventus); they do not know Terra's location (and if he's alive to begin with), they don't know how to bring back Roxas and Naminé (and if that's even possible) and they don't know or remember Xion. The top priority is saving Aqua and regaining the power of waking. If they save Aqua and Ventus, they have 7 guardians of Light and the clash can happen. Saving Roxas or Naminé is irrelevant when it comes to
defeating Xehanort, but it's relevant to KH3 in general because a lot depends on it (like I said before). There's no plot urgency in saving Roxas, that's "just" a personal desire from Sora developed after everything post-KH2. It's not surprising that,
after doing his part, Sora stops "actively" trying to bring Roxas back. All he could possibly do would be taking a KH science class so that he can either a) make a replica b) research the heart in order to find out how to release Roxas' heart without stabbing himself. It's prettyt obvious why none of this happen.
Differents plots can happen at the same time, you know. Sora is trying to regain the power of waking, Riku and Mickey are trying to find/save Aqua, while Ienzo (and later other people too) is trying to understand how to bring back Roxas and Naminé.
In KH3, Roxas quite literally gets dropped. Early on, we're led to believe that he's going to be one of the seven guardians.
How?
Axel hardly mentions Roxas, even when meeting Ven again. Nobody brings up Roxas' similarity in appearance to Ven at the Mysterious Tower.
This is literally what happens.
Even in the Keyblade Graveyard, none of the characters mention him.
Why should they, since Roxas has nothing to do with KG?
Of course, as I mentioned before, I think that relevance is subjective. The player decides (based on what they think about) how important a character is in the story. How important a subplot is.
It's definitely not subjective. If you cut it out and then the game falls apart, then it's relevant.
A perfect example: Are Maleficent and Pete in the game? Yes. Do they do their thing? Yes. Would KH3 be exactly the same without them? Absolutely yes, and this regardless of how the player feels about them.
Or: Are Cloud and Sephiroth, while part of the game (and not optional content like in KH1), completely irrelevant? Yes, KH2 would be the same without their scenes.
