I’ve never used Mendel, but this suggestion on Stack Overflow seems sane.

sudo systemctl set-default multi-user

You could also run get-default to see which target is currently being used.

You don’t have an account with Mastodon. You have a Mastodon account with sdf.org. Their mailing list that sent the email and the Mastodon instance they run are probably separate systems. You can forward your email to membership@sdf.org and ask to be removed.