Security Concerns with Private Repos

I have signed up my organization to a GitHub Teams free plan, and we are considering pushing our code to private repositories on GitHub. Our projects consist of decades old legacy code and there are lots of hard-coded credentials (not only in the code, but also in comments) for various servers and databases.

I do not want to make my team change all this code to store credentials in config files, I am not 100% sure our various tech stacks support this. It would also be very time consuming, and there is no guarantee we can find every single reference of credentials. I’m just wondering if it is safe to push the code with all these credentials even if the repositories we create are not public?