Isn’t just “x-risk” okay? Or is too much lost in the abbreviation? I suppose people might confuse it for extinction risks specifically, instead of existential risks generally, but you could write it out as “existential risks (x-risks)” or “x-risks (existential risks)” the first time in an article.
Also, “reduction” seems kind of implicit due to the negative connotations of the word “risk” (you could reframe as “existential opportunities” if you wanted to flip the connotation). No one working on global health and poverty wants to make people less healthy or poorer, and no one working on animal welfare wants to make animals suffer more.
Good point, ‘x-risk’ is short and ‘reduction’ should be or should become implicit after some short steps of thinking. It will work well in many circumstances. For example, in “I work with x-risk”, just as “I work with/in global poverty” works. Though some interjections that occur to me in the moment are: “the cause of x-risk” feels clumsy, “letter, dash, and then a word” feels like an odd construct, and it’s a bit negatively oriented.