If the main character in a novel disappears at the climax of the book, never to be heard from again, would that be a frustrating experience for the reader?
Details: classic three-act structure where the MC disappears at the climax, the remainder of the story will shift to the second most developed character who is almost as developed as the first, although you don't know what happened to the MC it is implied they are alive
I think it depends on the context. The disappearance should have some great importance to the plot, i.e. the protagonist may make a great sacrifice or battle that leads to this.
If you mean an unexplained disappearance that is never looked at/explained very little, that does sound frustrating if we have spent a lot of the book with the main character. I guess you should ask yourself if the disappearance makes sense, and about the nature of disappearance (e.g. we think they're dead and they make a dramatic return maybe could be interesting).
Explained disappearance can make a good story, even if some readers may be disappointed by this turn of events.
Unexplained disappearance will frustrate most readers and can serve as a cliffhanger for the next installment in the series.
Having the main character in your novel vanish at the climax of the book ought to frustrate your readers if you are doing a good job as an author. If it ends up not mattering to your story, in how it resolves, and readers are still engaged by the story at its conclusion, then it's fine.
If you have multiple main characters, ala George R.R. Martin, you can kill them off to up the stakes and make the story more dire and heartbreaking.
But, it does raise the question why was this character the main character of your story, since there seems that there is likely another character that was sufficiently developed and interesting who could be the main character and lived to tell the tale.
Also, it depends on what you mean by climax. If you are talking about the final 10% of the story, then that seems reasonable. Sometimes, in some story structures, the climax closes the 2nd Act. If this is the context you mean, then your alternate main character will need to be really well developed before the climax to keep the momentum of your story moving. Starting a huge character development 2/3rds into the story would likely seem tedious and annoying to me.
All other things being equal, yes. Could a great author make it work? probably. Would I recommend it? No.
It's hard to say more without knowing the context, and why you'd want to do this.
To answer your question: No, if the protagonist disappears before the end of the story it wouldn't just irritate the reader, it would likely not make sense, or maybe not even work.
I'll go into why below.
That your story seems to be carried by two characters might be because one of them is the main character and the other is the protagonist.
These terms are often used interchangeably to describe the "hero" of the story, but they mean slightly different things. Dramatica defines the difference as:
I.e. the main character is usually the carrier of perspective. In a first-person singular story, they'd be the "I". The protagonist carries conflict and is the one fighting the antagonistic forces.
A famous example of this split would be the Sherlock Holmes stories where Dr. Watson is the main character and Holmes is the protagonist.
Maybe one of your characters is the main character and the other the protagonist?
So, back to your question: do they all need to be present at the climax?
I think the most important part of the climax is the Climactic Moment. The Climactic Moment is the point in your story that, when it has ended, takes away all conflict and effectively ends the story. I.e. the Climactic Moment is the final showdown between the protagonistic and antagonistic forces. The Final Battle. (See the link above for examples where the genre is not "battle action"...)
After this scene, you can do some tying up of loose ends, and it's common to show "the new normal" but this could as well be done in an epilogue. Regardless, you don't spend much word count here. At most a few pages.
It is obvious if you're going to have a showdown between protagonistic and antagonistic forces, you need to have the generals of the respective forces present (I.e. the main antagonist and the main protagonist).
And if you've done your main character right, by this moment the reader is in sync with their feelings and thoughts, and removing this character from the most important event in your story would deprive the audience of the chance to experience it at top quality.
So I think all of the heroic functions, the main character as well as the protagonist, must be present in the climactic moment.
I'd even go so far as to say your main character/protagonist/hero should be present not only in the climactic moment but in the middle and the beginning as well. Even in the very first scene, preferably even the very first sentence.
So what to do if your story's protagonist or antagonist isn't present in the climactic moment?
The most obvious answer is to rewrite the story to make them present there.
But it could also be possible that the character you think is the main character/protagonist/hero isn't really. Otherwise, why would they wander off from the most important moment in their story?
Maybe some other character, present in most scenes of the story is the protagonist, and/or the main character?
Maybe the story can be rewritten to make that happen?
If you give the perspective and the role of Main Character to some character that is present in most of the book's scenes, and especially the beginning and the end, you could introduce a Dr. Watson that could make the story work without having to do so much about the conflict and the battle between the protagonistic and antagonistic forces.
Well, you may have to bump some protagonists up the ladder to make the protagonistic presence stronger in the final battle. And your current protagonist should then probably turn out to not be the strongest opponent to the antagonistic forces... I.e. you'd need to redefine who your main protagonist is and you probably need to plant that earlier in the story as well.
If this is a first-person singular perspective you'd have to cut all scenes where the character isn't present, or rewrite those scenes. But from your question, I surmise it's not, so then it's just about shifting the third-person perspective over to this character instead. (And one easy way to do that is to rewrite in first-person "I" and then either keep it or shift it back...)
It is also possible your story has more than one main character/protagonist, but in that case, it's preferable that their stories converge in some direct or indirect way in the climactic moment. E.g. one storyline causes the conflict and especially the climactic moment in the other storyline, or the different main characters/protagonists, having wandered in their own adventures most of the story comes together (by plan or serendipitously) to fight the antagonistic forces in the final battle.
