From the WordPress.org community team:
The next WordPress release (v4.9.6) is going to include a clear prompt to all users about upgrading the version of PHP on which their website runs. You can find out some of the technical information about this update in the core ticket: https://core.trac.wordpress.
If your server isn’t on the latest version of PHP, your website isn’t operating at its full potential. Keeping foundational server-side software up to date is a great way to not only help your own website’s performance but to help keep the internet, as a whole, more secure. Upgrade to PHP 7.2 today.
If you’re not sure which PHP version your site is running, be sure to check with your hosting provider.