The git checkout command is typically a local operation that does not require credentials. However, something has changed in your environment that git checkout needs to authenticate with the remote repository.
Large file support (git LFS) is the only case that I’ve seen where git checkout performs remote operations and requires credentials. I’m sure there are other cases (hooks installed into a repository, etc.), but that’s the only case that I’ve seen. If your repository now uses git LFS, then you’ll need to configure the checkout command to use LFS as well.