The short answer is it depends on the plugin.
Just because a plugin author didn’t update their readme file doesn’t mean the plugin hasn’t been tested and won’t work with the latest version of WordPress.
Sometimes authors of free plugins do not update their plugin files for each new WordPress release. They simply feel that if the plugin still works, then they don’t need to update it.
We offer guidelines on whether to install these plugins in our guide on installing plugins not tested with your WordPress version.