We should also never follow redirects, as this can pose a security risk. We expect users to always provide the final URL. We expect the validation in the admin UI to fail if it detects a redirect.
For example, if you provide https://www.gitlab.com/api/v4, the validation currently passes. But, GitLab responds with a 308 Permanent Redirect, because the actual URL is https://gitlab.com/api/v4. The validation should detect this, and fail.