The reason the Elf Boi had that option added was because a) he wasn't a combat encounter but was also b) not a particularly developed character, so there wasn't anybody who had any particular attachment to him, and c) he clogged the Old Forest encounter table without offering anything of his own (other than his sex scenes), and the Old Forest is already super dense with things like combat encounters and TF drops that we Actually Want. One rando with no personality who also didn't have a fight or item associated with him - the perfect storm for adding an opt-out.
By and large, combat encounters only disappear from the table if there's a plot reason for them to do so (such as the demon cultists being vanquished from the Rift after you blow up their lair in Winter Wolf). That accounts for Jen, the bimbugs, and the Frostwood Kitsune (also, you don't want the Frostwood Kitsune out of the encounter table because they're the best loot pinata in their zone). Ninian is also a combat encounter and he can be removed from the encounter table by advancing his plot: talk to him a bunch of times, move him to Hawkethorne, and then either build a house for him or ignore him. Vari leads to the Leothran Village, a location that is important for other content (like Cait's transformation and the leothran Christmas event), so that's not getting opt-outed because the player could inadvertently cancel something that they might actually want. Hatsumomo is the key source of wood in large quantities late in the game, so adding an opt-out for her would be Not Good. And Vaush's non-combat encounter is the only way to get yourself a virility potion so again, that's not something that we're likely to let the player accidentally opt out of.
Plus, most of those things don't happen in the Old Forest, which has the most comically overstuffed encounter table of any zone in the game right now and thus needs a way for people to pare it down if they wanted.
