A theme (component) honors .discourse-compatibility when it is installed, which is good 
However, when .discourse-compatibility is updated after the theme component has been installed and updated beyond the target version, the version is not rolled back on the next update of the theme component.
The only way to recover is to:
- copy the settings and store them in a safe place
- remove the theme component
- reinstall the theme component
- paste the settings
Which is doable, IF you know what is going on. But the regular “press the update button and see your problem go away” does not help you.
Desired: a theme component would inspect .discourse-compatibility on every update and roll back if necessary .
Yes, I know that can have nasty side effects but I’m convinced that those are less serious than not rolling back.