I m gonna do speedrun for resident evil 2 remake and i would like to know if is possible to use save for the Last boss or if it s gonna be deleate the run? Do i have to make one rush ?

You should ask this in the forum for the game. Much more likely someone who knows the answer will see it that way.

Typically, loading saves made during the run is permitted, loading saves made before the run is not. But neither this nor anything is a global, hard-and-fast rule. Definitely check with the game's mods/community.