Oh, This Comic: What If… #27 (cover by Frank Miller).
I was still behind on Uncanny X-Men, scrambling to fill in the gaps via back issue bins. I knew Phoenix had died in #137 but, though I’d read all the way past John Byrne’s run and into Dave Cockrum’s return, I still hadn’t found a copy of that issue & thus didn’t know the exact circumstances of her end.
Knowing I had X-Men fever, the manager of my comic shop suggested this issue — written by Jo Duffy, drawn by Jerry Bingham — as a temporary fix. I. Was. TRAUMATIZED. Pub date was 1981, so I probably read it in ‘82 or ‘83… and I’ve never opened it since.