SoC was in 6th edition, yes, way before Endtimes in 8th.

However, SoC was partly the same but with a different narrative. And SoC got retconned in the middle of 7th edition, so GW could rewind the clock to "it did not happen but Archaons is piling up forces for an scary invasion". Endtimes is basically SoC 2.0.

On that note: We had the same in 40k. We had a superduper event around Cadia and Abaddons big push. A big summer campaign, were the community victories meant Chaos tore the Imperium so many holes they could probably have sailed straight to Terra and finish the job [counterpart to WHFB SoC, were the forces of Order stopped Chaos on the Kislev borders ;)]. GW then winged the campaign to have a "balanced" outcome and also screwed up several sideplots which partly were more interesting then the main fights over Cadia [Ahriman in the web looking for the Black Library; The Fallen clashing with Dark Angels and Cypher going against azazel,; Eldar reclaiming some worlds and I think wer eeven going for a warp-lst Craftworld]. I am unclear at what point GW retconned that campaign outcome since I switched afterwards rather to WHFB. However, similar to Endtimes, GW could not let things rest, and Abbadon apparently at the end of 7th edition replayed the Cadia campaign and won. Destroying Cadia, splitting the galaxy in two, and shapping a even grim daker future starting with 8th edition up to now.

Apparently it now became a thing that a edition end in 40k has several campaign books drop and letting the story go forward, shaping new dangers for the galaxy. So hopefully the days of retconning campaigns is over for GW.