If you think someone is "staying dead", that just means they're taking longer than usual to come back.
Another running joke among comic fans was that "No one stays dead in comics other than Uncle Ben, Jason Todd, and Bucky." ...and two of those have since come back as well.
The X-Men (and related titles) were just more blatant about it. Going all the way back to the 60s when Professor X died at one point, only for it to be revealed years later that he secretly faked his own death, for reasons. But Jean Grey/Phoenix is the big one that sort of started the snowball rolling.