The docs mention that the security updates are for a repo.
I would like to know whether is possible to get updates for a particular branch. Often we need to update more than one gem and it would be nice to know how many dependencies are fixed while making such changes other than having to merge different PR with the master branch.
Unfortunately, it's not currently possible to get security alerts for branches other than the default branch for a repository, no. If you need something more real-time, you may want to look into security tools for the language or framework you're using. For example, npm has the npm audit command.
I hope that helps!