There's another community post with 404 debugging tips that might be helpful. Please give these solutions a try and let us know how it goes.

https://community.vercel.com/t/debugging-404-errors/437

Vercel Community

Debugging 404 Errors

Sometimes things don’t go as expected when deploying a website. If you see a 404 on your site, that means the page or resource couldn’t be found. There are...

A human should be around soon to offer more advice. But you can also get helpful information quickly by asking v0.

It looks like the domain is still connected to Vercel even if the domain isn't attached to any project anymore

To fully remove a domain from Vercel:

1. Go to your Vercel dashboard
2. Select the project
3. Go to “Settings” → “Domains”
4. Find your domain in the list
5. Click the “…” menu next to it
6. Select “Remove”

This will completely disconnect the domain from Vercel’s systems. Once removed, Vercel will no longer respond to requests for that domain, even with a 404 page.