As I see it the Abandoned Plugins option at present defaults to 365 days after installing MainWP. That means you will only see a warning a year after installing it for the first time. But the plugin or theme in question might actually have been outdated for years already.
There is a WP plugin called “Vendi Abandoned Plugin Check” which will add the period since the last update of each plugin in the WordPress plugin directory to the list of installed plugins, and will mark in red all periods longer than a year. It does so right after installation. It doesn’t check themes, though.
I think if the Abandoned Plugin function in Main WP worked the same way it would be far more useful right from the start. Especially if it worked for themes as well.