iOS Mail message won't delete on iPhone XR
I found a way to solve the problem without deleting the email account in Settings. As mentioned in a comment to my original post, deleting the email account was undesirable in this case because it is an account accessed entirely via another account (which required significant time to figure out how to set that up).

I was able to solve the problem by using the Safari browser on the iPhone to log into the gmail account where the problematic message was and deleting it via that browser gmail interface. Just to be sure, I also went to the trash there and deleted it forever.

I then quit the iOS Mail app by swiping upward and then upon reopening found the message was finally gone.

It's surprising to me that message was still there in the Safari interface. After all, I had had already deleted ("forever") the same message using the gMail iOS app.

I conclude that both the iOS Mail app and the iOS gMail app, though very convenient, do not provide the transparent control that the browser interface does.