> These emails indicate that those companies scrape people's Github activity, and if they notice users contributing to repos in their field of business, send marketing emails to those users without receiving their consent. My guess is that they use commit metadata for this purpose.

There are likely marketing email datasets floating around the internet that contain email addresses scraped from commit metadata.

I use a catchall with a specific Git client (not GitHub) email address, and found spam and phishing emails being sent there quite a few times.

May not necessarily be from commit messages, there's at least one way simpler way: simply adding .gpg to the end of any user URL will return that user's public GPG key.