Vercel’s deployment protection service is available on the Pro plans without any extra purchase, so might be suitable for your needs. The Advanced Deployment Protection is a paid service that includes extra features but we can’t offer a trial of this. You can see the difference between the offerings here: Deployment Protection on Vercel
Secondly, you can also create Vercel WAF custom rules to prevent website from accessing from specific IP, country, user agent based on your requirement: Vercel Firewall
Alternatively you’re always able to create your own password/protection service using Middleware. We have an example of how to do this here: examples/edge-middleware/basic-auth-password at main · vercel/examples · GitHub
Please let us know if you have any further questions or issues.