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composer.json PHP minimum Requirement => 7.x #691
From my point of view, there is no need to support PHP 5.x.
With Magento Core 1.9.4.x this package was included in Magento Core
Besides that: We can strongly recommend using PHP7 at least for performance reasons
My vote is for 7.0 to be the minimum since it isn't that old and is still supported by Ubuntu 16.04 LTS for two more years.
Debian 8 still uses 5.6 and will not be EOL for 11 more months, but there is an upgrade path to Debian 9 which uses 7.0.
Of course, for people using Docker (which I advocate should be everyone) which distro you run doesn't matter. The official docker images actually stopped supporting 7.0 already with 7.1 being the oldest getting updates.