Over the years we’ve gone from always showing all plugins in searches to devaluing plugins that aren’t updated in a time span to devaluing them if they’re not compatible with the latest few releases of core. All of this is done to improve the user experience and to ensure they only find plugins that are actively maintained and compatible with the versions of WordPress they use.
As part of this, when a plugin is closed we currently require the ‘tested up to’ value to be, at least, the latest stable version of WordPress core. We have updated our emails for closures and re-scans to reiterate that, but it’s for a slightly different reason than helping users.
We want to help you developers. If no one can find your plugin, because it’s not compatible with (say) WP 5.5, then no one uses your plugin. Presumably, if your code is hosted here, you want people to use it. To help you and ensure your plugins can be found and used, we are requiring you update that, should we have any reason to close your plugin.
Just like you have to bump the plugin version so people get notified of updates, you need to make sure that “tested up to” value is current 🙂
So! Please keep that up to date! It’ll help people find your plugin, give them confidence in your work, and help make you more successful! Wins all around 🙂