I installed many of my WordPress sites using Softaculous.
All my sites that I had transitioned to ClassicPress have just been updated to Wordpress 5.5.1 because of the auto-update setting in Softaculous that I forgot to turn off.
There are three auto-update features in Softaculous: program, plugins and themes. I am going to turn them all off to prevent possible problems in the future, and take care of the updates myself.
The revert is not the end of the world, but it will be a lot of work to do all my transitions over again.