Slightly separate though maybe related issue: any understanding or speculation why Apple has not implemented tags in Mail.app? Arbitrary tagging was introduced in the Finder way back in
MacOS X 10.9 Mavericks (2013). Ever since then an obvious next step would be enabling the same, extremely useful functionality to help manage and organize email.
Gmail added support for arbitrary tagging about a decade ago (a quite excellent, flexible implementation, albeit completely non-standards-based). And just last week Apple announced support for arbitrary tagging in the upcoming macOS 12/iOS 15 versions of Notes and Reminders. But still Apple is apparently holding off on tagging for email — there appears to be no attempts to either work with Google, collaborate with email standards bodies to make this possible, or roll its own, proprietary solution.
* correction: Apple introduced arbitrary tagging into the Finder in 10.9, not 10.7 as I originally thought.