It’s not yet resolved. Until SvelteKit changes its adapter to use the Functions API v2: kit/packages/adapter-netlify/src/serverless.js at main · sveltejs/kit (github.com), this won’t work.
An alternative would be to use the connectLambda function: netlify/blobs: A TypeScript client for Netlify Blobs (github.com) which can be called anywhere in the Lambda code as long as you can pass the event. The Svelte adapter provides you access to context but not event: kit/packages/adapter-netlify/src/serverless.js at main · sveltejs/kit (github.com).
A “hack” would be to add a postinstall script or something similar, that can modify the adapter’s code to add the connectLambda function in there. I’m not sure if adding this feature to the official adapter works or not, as it might or might not be needed by all.