I mean, if you're playing a Dungeons and Dragons universe those things probably exist somewhere as NPCs and could theoretically be around. Harengon are from another dimension so that's an easy in or out depending on the Feywild/Plane of Faerie being around, Plasmoids are from space, Firbolgs are regular creatures who also have that feywild link. Have you just, never had any of stuff like firbolgs show up? Shardminds are another 'they're from space' so that's another reason to be rare but yuan-ti are such stock D&D creatures that they even appear in Dark Sun. Do you just, not have secretive snake cults, the most swords and sorcery thing ever? Not as playable races (as, frankly, you don't want my options on your playable options as the only interesting thing there is gnomes and you seem to have put down 4 seperate instances of Human but X), but not even as NPCs around the world?
If your setting doesn't have firbolgs, long time legacy giant-kin, or yuan-ti, long time threats with entire D&D video games based around them, both of who've been around since Monster Manual 2 and have stats in every edition as monsters, I gotta once again bring up that 'why are you using Dungeons and Dragons in particular' because like. These aren't exactly rare creatures. Monster Manual 2 gave us Driders, Duergar, some of the more regular re-occuring devils and demons, and, once again, Krakens. Does your setting not have those either? What's the cut off for when you stop using the monster manuals?
If you don't want to play a kitchen sink then maybe playing Dungeons and Dragons, the kitchen sink fantasy setting, isn't for you
I mean, yes, the players do decide what options are allowed through what they chose. If you have some fancy home-brewed option that has massive importance to your lore and you've offered as an option, it means exactly squat if no one picks that option and decides "you know what its gnome time".
You'd have missed Mystara and X9, the stuff when they appeared