Tagging email messages in Apple Mail
Though mail tags will no longer be supported, and though there is no tagging feature in Apple Mail, I have a solution that works for me and my workflow.
I use the program called "Things." Although it is a to do program, it has a quick add function that works beautifully with mail. With an email message selected, just hit the quick add keystroke and it instantly creates an entry into things, pulling up a window that allows you to tag it, along with additional categorizing options, due dates, display dates, folders, etc.
It's all very instantaneous and includess a link to the original message in mail. So in other words, if you search for the email in "Things", you can click on the link and it will open in mail.
Here is the link for "Things" in the App Store:
https://apps.apple.com/us/app/things-3/id904280696?mt=12
Though I am not affiliated with THINGS, I have used it for many years and find it to be the best App. I use it as part of my daily workflow.