After installing the MacOS Big Sur upgrade, I was dismayed to find that the beloved red flags in the left column of Mail, which I'd used a lot to indicate messages I needed to do something with, had disappeared.
When I clicked on "Flag" in the menu bar, a flag would appear in the email message itself, but not next to it as was the case before Big Sur.
Thankfully, a bit of Googling revealed a simple fix. Just right-click the "Subject" box near the top of the screen and make sure "Flags" has a check mark next to it.
Once it does, voila, the flags appear as before. Here's a screenshot of some flag-happy messages in my inbox.