Theory: There are no quirkless people
Here’s an shower thought I’ve had for almost a week, and I needed to get it out, and hear other people’s opinion.
There's no such thing as a quirkless person, it's just their quirk is unintelligent/unusable/hard to find during normal life, and just doesn't appear to have quirks.
(Quirkless people were probably originally people who haven't generated their quirk yet, and many people say that the quirk wasn't fully spread onto, and the people who didn't get spreaded on was quirkless. However quirks are said to be the next stage of evolution in the human race (comfirmed in the manga and anime). hence every human should be, as every giraffe has a long neck.)
(It's been 100 something years, how will there be people who hasn't gotten the quirk-like virus? Deku's parent's both had quirks as well, so it'd hard to think he didn't get an quirk, which goes for all the other quirkless people as well, which is why quirkless people are so rare, so it just seems a bit weird to me)
For example, Toga's quirk would've never been found, If she never ate an bird or got looked into, no one would've guessed her quirk. No one would've made her drink blood.
Therefore, society would consider her quirkless.
One For All is also an good example. He/They were created because somebody who thought they had no quirk was then given a quirk, but then later found out that he in fact, had the quirk to give quirks. The reason why quirks are able to pass on is because people who are considered quirkless never had any chance to use their quirk, so their body never fit it into that quirk, meaning a new quirk would be able to replace/over power it