Hi,
Right, the settings menu is accessible from the plugins screen or from Plugins -> Test jQuery Updates.
However this plugin requires a specific version of WordPress to work properly. Version 1.x of the plugin is for WP 5.5. Version 2.x (latest at the moment) is for WP 5.6.
Unfortunately the code in version 1.x didn’t show any messages/warnings when the plugin was incompatible with the WordPress version (i.e. when WP was updated and an old version of the plugin was still active). It just disables the plugin to prevent errors/bad side effects, hence the menu disappears.
Now in v2.0 there is some code that will show a message on the plugins screen and on the plugin’s settings screen for next time.