The official stance of WordPress.org is that WordPress is supported on PHP 5.2.4 or greater.
The official stance of the Plugin Team regarding what version of PHP your plugins can use is .. not that.
We don’t have an official stance. We’ve never needed one. We do (often) test complex plugins on multiple versions of PHP (and sometimes HHVM) to make sure there’s proper degradation and support, but at the same time, we do not have an official requirement that you must support version X or Y.
This is not an official requirement post.
This is a reminder post.
Then Mika Epstein goes on to show a variety of ways to provide notices for required versions of WordPress and PHP in the post.
This will be familiar to most folks with big plugins already, but if you are newer to writing plugins, then Mika’s reminder will be helpful.