I think retiring Sapper was the right decision. I have a feeling Rollup will meet a similar end too.
They both seemed like fine pieces of software. They just don't seem offer separate much their counterparts nowadays that warrants continued feature development.
Rollup isn't going anywhere! When you poke behind the scenes of newer build tools, like Snowpack and Vite and the custom compiler that powers Remix Run, you'll find Rollup doing a lot of the heavy lifting. And it's the preferred choice of most libraries. There's a reason for that :)
The project hasn't really been publicly released yet. The repository is still private. The package is up on npm, but isn't ready for widespread adoption yet. I'd recommend sticking with Sapper for now.
I am not sure I buy into all the svelte hype, and even if claims about it's speed/build size are true - the difference isn't enough to outweigh the enormous advantage vue has in terms of a more mature ecosystem/tooling.
https://sapper.svelte.dev/