In Big Sur, you can create an Apple “Reminder” with a link-back to the email message that pertains to the Reminder item.
In Mac Mail, highlight the Subject line of an email in Mac Mail, then right click (or Ctrl-Click), and, in the pop-up menu choose Share > Reminders. A window pops up with a Reminder item consisting of the (editable) subject line of the email, and it allows you to choose which Reminder “List” to add the item to. The item that gets added to Reminders has a Mail icon. If you click the Mail icon from the Reminder item, it links-back and opens the original Mail message that you shared to Reminders.
Here’s the Apple Support article: