Warning: Spoilers for My Hero Academia Season 6, Episode 18How Deku got his power has been a tightly kept secret in My Hero Academia, with only a handful of people aware of One for All and its ability to be transferred. Now that Deku has a target on his back, though, the secret is out, both among his friends and the public, although each to a different extent.
Due to the sheer number of heroes involved in the fight with Shigaraki/All for One in My Hero Academia, it came as no surprise to learn that some had heard the term "One for All" without knowing what it meant. It was reported to the media and the public that Shigaraki was after something called "One for All," and people began to panic about what this might be. Hawks and Best Jeanist come to Endeavor to ask, who says it's related to Deku. Realizing that his being a target may put the others in danger, Deku takes off on his own after awakening from a coma in the hospital, leaving a note for each of his classmates explaining what One for All is, how he got it, and why he has to leave.
UA's Knowledge of One for All
Those at UA who received a letter from Deku, which seems to be all of Class 1-A, now have the full story on Deku and One for All. He explains that he was born Quirkless and that the power he now has was passed down to him from All Might. He also explains that One for All is what All for One wants most, and thus being around him may put them all in danger. The only one who already knew was Bakugo, of course, as Deku had told him the truth much earlier.
What does Class 1-A knowing about One for All change? Probably not as much as you might think. They now know there is a direct threat against Deku in particular, and that he's left to spare them. They also finally understand why All Might and Deku seemed to have such a close relationship--he's literally All Might's successor in My Hero Academia. What Deku forgot to take into account here is that he's written this note to a class full of future heroes; they're not just going to let him do this alone, even if it is what he wants. By telling them all, Deku has effectively invited them to come after him.
The Public's Knowledge of One for All
While the public know less about the situation, they know just enough to cause more trouble. The less scrupulous members of the public may decide that the best way to protect themselves from All for One/Shigaraki is to capture Deku and turn him over. While the average civilian is likely no match against Deku at this stage, many people do have support items now, and in large numbers could be enough of a threat to take on Deku, if so desired. That would put Deku in a very tough spot, as he would surely be unwilling to hurt civilians even in self-defense. Even without directly confronting him, though, there could be public pressure on the heroes to abandon Deku and turn him over, and their refusal to do that could hurt the public's support of heroes even further.
The secret of One for All getting out in My Hero Academia could ultimately play out in a number of ways, but all of them are going to make things more difficult for Deku.
New episodes of My Hero Academia air on Saturdays, via Crunchyroll.