Hi,
The minimum required version of Joplin before upgrading a plugin is not always mentioned in the plugin repository. Since there is no warning message in the plugin manager (referenced bug), I thought I could update Rich Text Markdown, wrongly. Now, the plugin does not work anymore, neither does the 0.8.3 version I had saved.
To get this extension working again in its 0.8.3 version, I renamed the "plugins" folder, created a new empty folder, which allowed me to reinstall my plugins (only 3), including Rich Text Markdown 0.8.3.
Yes the bug is extremely annoying that can be updated to an incompatible version.
If the creator of the plugin provides the versions, they can be easily downloaded and installed. If not, at NPM or also in the oficial repro you can find older versions, but this is more complicated.
Download the JPL package in the desired version
Close Joplin
Copy the downloaded JPL package in your profile plugins folder