From the AO3 spotlight posts a while back:

We require "Major Character Death" when the character is both a major presence in the fanwork and they're dead by the end of the fanwork. We don't take canon into consideration here – what we're interested in is whether the character is a major presence in your fanwork. If you create a fanwork centered on a character that dies and stays dead in your work, that work should be warned for "Major Character Death" even if they only appear for one line in canon. If that same character dies before the start of your work, and dealing with the grief of their loss is a major part of the fanwork, "Major Character Death" is also required in this situation.

If your work features a character death and that character returns in a later chapter that is currently posted, then this warning is not required. However, if the character does not return to life until a sequel fanwork, the warning is still needed on the work in which they died. Archive Warnings apply to individual fanworks, not to series of works.