Old apps you no longer use (also called zombie apps) can present a security risk because you might not have updated your privacy settings on them. Android users can go into the Google Play Store, select “My Apps and Games” and sort them by “Last Used” to see the oldest ones.
iPhone users can go to Settings, then into “General” and “iPhone Storage” to see the date they last used an app. Before you remove the app, though, go into it to delete or disable your account that contains all your data.