Hello,
I see 70k+ active installations, so even though the Classic Editor isn’t supported anymore, people upgrading to newer WP versions (such as 5.8.1) will likely continue to use this plugin if they still use the Classic Editor. I do use it on some installations since users are just not accustomed to the new editor, and I would have to deactivate the plugin if upgrading to newer WP versions would break them.
From what you wrote it seems that the WP version is less relevant in assessing compatibility of this plugin as long as the plugin works with the Classic Editor, hence if the Classic Editor works with a WP version, this plugin would work as well.
Thank you