News Courtesy of ZDNet.com:
The WordPress open-source content management system (CMS) will show warnings in its backend admin panel if the site runs on top of an outdated PHP version.
The current plan is to have the warnings appear for sites using a PHP version prior to the 5.6.x branch (<=5.6).
The warnings will contain a link to a WordPress support page with information on how site owners can update their server’s underlying PHP version.
In instances where site owners are running their WordPress portals on top of tightly-controlled web hosting environments, the web host has the option to change this link with a custom URL pointing at its own support site.
The warning will ship and start appearing with WordPress 5.1, scheduled for release early this spring.
Ryan’s Take 
There is no reason NOT to upgrade your PHP version to 7.x. It’s faster, more secure and has features that some plugins and themes require anyhow. Perhaps the only websites running a lesser PHP version are those that haven’t been updated in ages. If that’s the case, there is a good chance that the theme being used has been abandoned which would necessitate a redesign.
I’m surprised that the WordPress team didn’t set the warning level to a minimum of PHP 7.0. For those of you wondering what happened to PHP 6, it was skipped. The reason being is that the changes and improvements introduced in 7.0 were so significant that the team decided it best to change the naming structure.
If you’re worried about making the switch to the newer version of PHP, the best thing to do is contact your hosting provider. Many web hosts offer a control panel where changing versions can be done with a setting. If the site breaks, it is easy enough to change back. Of course, you’ll want to make sure your core WordPress, plugin, and theme files are all up to date before proceeding.
