This is pretty much the same issue as TXT Verfication. In addition to is-a.dev this happens to other users of the following subdomain services which are listed on the PSL as well:
- GitHub - open-domains/register: Free subdomains for personal sites, open-source projects, and more.
- GitHub - is-a-good-dev/register: Register for your is-a-good.dev domain!
If possible adding full support for the PSL to Vercel would be amazing and would help many subdomain providers that are listed there. Basically allowing subdomains of domains listed on the PSL to be used across many user accounts without verification of the root domain. This functionality is already included in other services similar to Vercel such as Netlify.