Yes, I'm talking about "git send-mail" and "git am". Actually, it's relevant to what you're already saying. You can have an e-mail list based code review process, and store it in your on-premise servers, or in your infrastructure for what matters.

You can argue about the utility and practicality of GitHub, and the tools provided by the platform, however Git is neither is an invention of GitHub nor have a monopoly over it.

Lastly, having no clue about a technology as a professional developer has nothing to do with the utility or usefulness of the feature. On the contrary, a professional developer should be able to work in a much more flexible and open-minded manner about different working methods and workflows.

All in all, the mailing list can act as an internal time-based knowledge base which can store both the code review and decisions coming out of it. The decision to use it or not of course yours and your team's, but send-email and am are not features confined to "professional OSS committers who aren't working on projects hosted on GitHub" by any means technical and social.

Also, you can like the feature or not, and may decide against it just on personal preferences. However, this neither moves the feature to a niche place, or makes it obsolete.