Vercel does not have that feature at the moment. But the vercel domain can be assigned to another branch and this way you can switch your environment and make your productive enironment unreachable. Before you need that, you need create a branch in GIT that serves a "website in maintanance mode" site.

You can assign a different branch in Vercel this way:

1. Project > Settings > Domains > Your Domain > Connect to environment > Preview > Branch > Select a branch that serves a "maintanance site"