WordPress in the upcoming release 5.3 now includes a feature to verify the admin email every 6 months. The idea is to make sure that the email set as the admin email is valid and still an email address that is being used for the site.
Having the admin email verification show every 6 months will be something that could be annoying to clients when they go to log in to wp-login.php on client sites. But in WordPress core in release 5.3 there is a filter include so that you can both change the month time to a different value in this example setting that to be every 9 months.
You can use this code snippet to set the admin email check interval as every 9 months from the default of every 6 months.
With this code snippet, you can completely disable the admin email check from happening.
add_filter( 'admin_email_check_interval', '__return_false' );
Being able to easily disable or change the admin email verification interval will help your clients not have to run into issues on the client sites.