Doesn’t work on WordPress 5.3.4

It works on my test site https://lesterchan.net/wordpress/2011/04/28/wp-showhide-1-00/ on WP 5.4

If it doesn’t work that means you have a plugin conflict on your end.

How can I understand what plugin makes a conflict with yours?

Sorry – I found the problem, it was really on my end.
But watch out, please, that there is a mark on the compatibility of your plugin with the latest version of WordPress

Yeap, because I have no time to bump the version yet.