This is blocked on: T172165: Require either PHP 7.0+ or HHVM in MW 1.31
Certain versions of HHVM have revealed issues with data types in MediaWiki (e.g. T163646, T140878, T126871, T177134, T140864). Although PHP 7.0 is likely to hide those issues from view again, as does PHP 5.6, I think using type hints in function declarations (PHP 7 supports type hints for scalar types) will make code more robust, will force repayment of technical debt (rather than using hacks like https://gerrit.wikimedia.org/r/381617, https://gerrit.wikimedia.org/r/302430 or https://gerrit.wikimedia.org/r/381616) and in the future, possibly, will lead to gains in performance (as does strict typing in HHVM when using repo authoritative mode).
I therefore suggest a goal more ambitious than merely dropping support for PHP 5.* (task T172165): to strongly encourage (preferably, using some technical means) strict typing of function parametres and results in MediaWiki code, using <?php declare(strict_types=1); wherever possible.
This task also implies raising PHP minimal version to 7.1 in the near future; as only 7.1 allows nullable parametres.
This requirement can be enforced by automated testing, as far as I understand.