The plugin provides an expandable section in blog posts.
Fortunately WordPress now supports this natively via the Display block type. Some people are hunting around for other plugins with additional functionality, but Display looks good to me.
One issue I had was finding which blog posts used Collapse-o-matic.
The solution I found was in the WordPress admin posts page, search for [expand
Then it’s hopefully easy enough to move the relevant content into a Display block, and deactivate the plugin.
For anybody else using WordPress, this is probably a nice reminder to review WP plugins and check for any other vulnerabilities that haven’t been widely advertised.