Elements like
ancestries,
backgrounds,
classes, and
heritages that a player must select at character creation can still be uncommon or rare. Obviously, there's no opportunity for the player character to search for them during play, but these rarities still indicate the prevalence of adventurers with those elements in the world. You can decide to allow them on a case-by-case basis depending on the campaign and the story your group wants to tell. For instance, a game set in the Szandite Collective might have fonqugon as a common ancestry, while the typical common ancestries are less common. An official player's guide for a Starfinder Adventure might have uncommon backgrounds that you can access by playing the adventure.