Warning: contains spoilers for My Hero Academia chapter 335!
As My Hero Academia went through its villain roll call in chapter 335, one particularly bizarre-looking Near High-End Nomu raised some uncomfortable questions about the ultimate fate of Japan's #3 hero, Best Jeanist. Nomu are artificially constructed beings which typically wield multiple quirks, and work in the service of main villain All For One. The original nomu were shown to be very unintelligent, ceasing movement once they were no longer receiving orders, but both earlier prototypes like Number Six and Kurogiri and the later (Near) High-End Nomu are more self-sufficient, with the one that battled Endeavor even capable of limited speech. One of the most horrifying aspects of the Nomu is that they're built using a dead body as a base, as with the tragic scenario of Shirakumo/Kurogiri.
When Hawks was trying to infiltrate the League of Villains prior to the start of the Paranormal Liberation War, he turned over what appeared to be the dead body of Best Jeanist to the villains to earn their trust. Best Jeanist was absent for a very long time, and didn't return until Dabi attempted to reveal that Hawks had "killed" him on live television. This confirmed that Best Jeanist had been a willing participant, pretending to be dead to lend Hawks some credibility. Yet, this still means that Best Jeanist's body was in the villains' possession for months - villains like All For One, who is prone to stealing quirks and who might not fall for this trick.
One of Best Jeanist's most distinctive characteristics is that his costume conceals much of his face, making him look as if he has a very long neck - a trait that's been continually exaggerated as the series has continued. One of the six Near High-End Nomu bears a strong resemblance to Best Jeanist, with an extremely long neck and an exposed brain where Best Jeanist's hair would be. It's possible that after the villains took custody of Best Jeanist's body, they realized he was still alive, and decided to use him as a base for one of the Near High-Ends.
The theory may seem a bit far-fetched, as Best Jeanist has reappeared in the manga as of chapter 291, but there's another piece of the puzzle which offers a potential explanation. Himiko Toga, whose quirk allows her to transform into anyone whose blood she has access to, has been absent from the story (aside from one or two single-panel appearances) since chapter 289, following a fight with Ochaco. Since Toga's quirk evolved to copy a person's quirk as well as their appearance, it would be extremely difficult to catch on to her disguise. The villains could've farmed Jeanist's blood to create a reserve for Toga, then turned him into a Nomu. This would create an exceptionally dramatic reveal for Bakugo, as the hotheaded young hero is Best Jeanist's protege, with the two associated as heavily as Deku and All Might. And it wouldn't be the first time that a character was revealed to be impersonated after a long duration, since both Toga and Twice have been responsible for this before.
While Best Jeanist may well still be alive, the villains had plenty of time to take, study, and duplicate his quirk, so even if Best Jeanist isn't dead, the Nomu that bears a resemblance may still be in possession of a copy of his quirk. And, being a top-tier hero, Best Jeanist's quirk is extremely powerful, and could be quite a deadly threat on its own. With the final arc of My Hero Academia currently underway, these Nomu will need to be dealt with soon, so the answer will arrive before long.