Hi, @kostas-pblworks! Welcome to the Vercel Community.

There's no built-in Vercel feature to automatically fallback to another URL or serve another branch when a build is skipped due to the "Ignored Build Step" setting. But here are some suggestions:

1. Conditional Rewrites: Instead of static rewrites, you could try using Vercel's Edge Middleware. You could check if the target deployment is available before doing the rewrite so you'd fall back to the current project if the target isn't available.

2. Deployment Hooks: Have you considered setting up a deployment hook on the project that gets rewritten to? This could trigger a deployment on the other project whenever a successful build occurs, keeping both projects in sync.

3. Shared Build Configuration: Maybe you could refactor your monorepo setup to have a shared build configuration? This could ensure both projects are built together, reducing the chances of one being built while the other is skipped.

4. Caching Strategies: You could implement caching at the edge to serve stale content temporarily if the target project isn't available. Vercel's Edge Config or a third-party caching solution could help here.

5. Health Checks: Implementing health checks in your apps could be useful. Before doing a rewrite, your app could check if the target is healthy and fall back to a default behavior if it's not.

Hope this helps! Let me know if you have any other questions.