As revealed in Blue Lock chapter 298, Seishiro Nagi was locked off following his poor performance for Manshine City in the Neo Egoist League. However, given the character's significance in the story, all fans can wonder is whether Nagi still has a chance to return to Blue Lock following his disqualification.
While there is a chance for Seishiro Nagi to return to Blue Lock, it is not set in stone. While Ego Jinpachi has always painted the project as a high-stakes training program where disqualification meant "death," it has been noted that the rules aren't as strict as players have returned following their "disqualification."
Disclaimer: This article contains spoilers from the Blue Lock manga and reflects the author's opinion.
Seishiro Nagi can still return to Blue Lock
As fans may remember, as Nagi and Reo were underperforming in the Neo Egoist League, both strikers were worried they would be locked off if they did not perform well in the tournament's final matchday. While Reo Mikage managed to score a goal and get an assist, Nagi failed to score a solo goal despite getting fired up.
With that, Seishiro Nagi ended up with only one goal contribution in the Neo Egoist League. This one goal contribution was the one he scored against Bastard Munchen using his Five-Shot Revolver Fake Volley. This goal was also why he was ranked high at the initial stages of the Neo Egoist League.

Nevertheless, despite his best efforts, Seishiro Nagi failed to score a goal or get an assist in any of the remaining matches. This development saw him end up ranked 24 in the Neo Egoist League's Final Auction Rankings. As fans must remember, only the top 23 players would play for Japan in the U-20 World Cup, meaning that Seishiro Nagi had gotten locked off.
While there is a chance that we may likely see the last of Seishiro Nagi in the upcoming chapters, there lies the possibility that he might return. Ego Jinpachi has always tried painting the picture that the Blue Lock project was a high-stakes training program where defeat meant "death."

However, this policy was evidently never set in stone. As fans must remember, while Kunigami Rensuke was "locked off" after his disqualification in the Second Selection, he returned to the training regime after surviving the Wild Card.
In addition, while it was stated that Blue Lock would take over the U-20 team after they won against them in the exhibition match, Oliver Aiku, Sendo Shuto, and the remaining Japan U-20 team ended up joining the Neo Egoist League alongside the Blue Lockers.
So, considering that significant players who were seemingly ejected from the series have noticeably returned, there is a chance that the manga might reveal such a development through which Nagi can join Isagi, Reo, and others as part of Japan's U-20 team.

One possible theory is that Ego Jinpachi might pick a few extra players other than the top 23 to encapsulate the Japan U-20 team. This is because, while national teams are only allowed to have 23 players for a matchday, they can call up 26 players as a failsafe in case of injuries or illnesses. In such a case, a national team is allowed to switch players even during the tournament.
Moreover, fans should remember that, while Ego Jinpachi stated that only the top 23 players would make up Japan's U-20 team, Sae Itoshi was never part of the Neo Egoist League. As any fan would know, not calling up Sae for the U-20 World Cup squad would be a foolish decision by Ego Jinpachi.
So, despite what he said, there is a likely chance that Ego Jinpachi could still make some last-minute changes to the squad before the World Cup begins. Thus, Nagi being "locked off" could effectively be a development meant to help him become an even greater egoist.
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