And oh, hey, Spock has . . . a brother. TERRIBLE when they do that.
Like, oh, hey . . . Dukat had an affair with Kira's mother.
Or when Picard had a brother all of a sudden.
Or when Riker has a father he doesn't get along with.
Or when Indiana Jones has a father he doesn't get along with.
Or when Trip has a never before mentioned sister.
I really don't get what problems people have with Sybok being Spock's half-brother he never talked about.
Do you want all characters to be introduced with a complete and final bio in the very first episode? That's not how it works.
Spock's father, mother and fiancee where introduced just like Sybok. All of a sudden, they were there. They haven't been relevant to the stories coming before, which is why they were never mentioned.
At which point in TOS or the TOS movies could it ever have been adressed that Spock has a half-brother who was born long before Sarek met Amanda?