Sat-Yr-9 was always an odd subplot. I had always assumed Courtney would turn up at some future point having been transported to Sat-Yr-9's cell or somesuch, so killing her off so abruptly was a surprise in retrospect. But then Chris Claremont had a number of dangling subplots from his initial years worth of stories that went unanswered for a long long time, indeed some have never been resolved!
But with Sat-Yr-9 I wonder if the intention was to set her up as Excalibur's covert ongoing nemesis, a more dangerous version of Vixen who operated from the shadows and would control most oof the British underworld and power structure. We saw a hint of that in her visit to the British Hellfire club.
If I recall correctly Courtney/Sat-Yr-9 was still spinning her wheels in London as late as Claremont's X-Treme X-Men run... quite a remarkable loose end to leave.
Not X-Treme X-Men
It was during his Uncanny X-Men
run, starting around #450 (give or take). Claremont also used her a little in the Genoshan Excalibur
, and later New Excalibur
(where Brian was sort of dating her while he mourned Meggan).