Model must be downloadable by the public before March 1, 2024.
If there is a flagship image model released by Stability and they decided to name it differently than Stable Diffusion, but it is clearly in the spirit of an upgrade to Stable Diffusion and meant to be its progression, it will be counted as YES.
If results are sufficiently unclear, the question may be voided.
People are also trading
For clarity, a comment by Stability staff:
https://www.reddit.com/r/StableDiffusion/comments/1ax8c2x/comment/krqncaj/?utm_source=share&utm_medium=web2x&context=3
SD3 is the successor to SDXL. Cascade is a secondary research model. It has some really cool tech in how it was built, but isn't really part of the mainline for real usage.
I believe "Stable Cascade" would qualify.
Announcement: https://stability.ai/news/introducing-stable-cascade
Repo: https://github.com/Stability-AI/StableCascade
Model download: https://github.com/Stability-AI/StableCascade/blob/master/models/download_models.sh
In the announcement, it's compared to SDXL and SDXL Turbo.