Gojo Satoru's fate in the manga is indeed death. This event has significant implications for the story and the characters around him. It changes the dynamics and raises the stakes for the remaining plotlines.
In the Infinity War comics, not everyone is permanently dead. Resurrections and plot twists often occur. Different story arcs and continuities can change the fate of characters.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
Well, usually it's indicated by clear plot points like a final confrontation or a lack of further appearances. Sometimes, it might be stated outright by other characters.