The third bullet point at https://docs.github.com/en/site-policy/github-terms/github-t... --
> One person or legal entity may maintain no more than one free Account (if you choose to control a machine account as well, that's fine, but it can only be used for running a machine).
This is the primary reason why I have only ever used my own personal account for work stored at GitHub. If a company is expecting its employees to use a separate work GH account, one assumes that company is planning to pay for all those accounts for its employees.