You'd like to get in touch with a GitHub user but, e.g., you don't want to post your question as an issue because, e.g., it's not about a specific repository but more generic.
As a result, the problem can be translated in How to find the email address linked to a GitHub account.
Recently, I had this need in order to contact someone with a Docusaurus blog offering a feature that I couldn't find documented on the web. His blog was not on GitHub but other repos well.
Indeed, GitHub provides a.o.t. an option to make the email private in his user settings page.