Yes, you're right. Despite I did ton of tests to adapt the extension to PHP 7.4 (and now to PHP I didn't get the alert because my environment was not configured to show DEPRECATION notices. If do you have that setting in your live site I think you should change it to avoid tons of extra logs.
Anyway I have modified the plugin to avoid this message in a a future. I attach you a modified version; just download and uncompress the file and overwrite the file located into /plugins/system/securitycheck folder.