Oh, I still need rednotebook in order to ever have any significant hope of finishing my 2 novels in progress. My original hope in posting a question to this site was that someone could guide me to a tutorial on how I can run rednotebook from the ubuntu ppa on Debian that is not too complicated for me to use. Of course, I found lots of ppa tutorials, but they’re all complicated and like with any tutorial found solely by searching, there is no guarantee they will actually do what their authors claim on my system.
I guess I should try posting this to Linux Questions. I guess I need to rework my question to make it perfectly clear that updating to Mint 20 is not my first choice about how to get around this problem. Completely dumping all versions of Mint and going back to Debian with a fully functional rednotebook is my first choice. Mint will never be my first choice because it is downstream from Canonical.
And I used to be on the mailing lists for various Debian emails, several of them, and your account of how rednotebook came to be unsupported in Debian does not line up with what I read in those emails at the time.
As I remember it, Debian had a vacancy and needed someone to take over maintaining rednotebook, and so they asked for volunteers who might be interested in taking over. They got no volunteers, so they stopped maintaining rednotebook, and it could no longer be downloaded for new installations from the Debian repositories.
Rednotebook worked just fine for me for quite a while after this, but gradually it began to freeze up for a minute or two every time after it saved. I never figured out how to stop the automatic saves, so I was constantly being interupted, and it eventually became very difficult to write with rednotebook.
So I installed Mint in one of the partitions in all my computers. But now after Mint has become so pushy about upgrading it is definitely time for me to dump mint. And in the future I will follow my personal rule of never using an OS that is downstream from Canonical because what Canonical has done to Debian has wrecked it.
Thanks to everyone for all the attempts at advice.