[Taken T216165#5284682 by @Krinkle]
I'd like to try to swing this around and instead look at what the highest PHP version is that we could require, for MW 1.33/34 (regular stable) and MW 1.35 (LTS). [Per the review below we're looking at PHP 7.2 for MW 1.34, but we'll want to use the same criteria in the future for the next major update.]
This exercise makes the following (potentially controversial) assumptions:
* We prefer not to support a PHP version beyond its EOL.
* We prefer not to support an Ubuntu or Debian version beyond its EOL.
* Each Ubuntu and Debian version must have at least one MW version that we still support when the Linux release cycle starts.
* Administrators using the Ubuntu and Debian LTSes must have a clear upgrade path from one combination of Linux/PHP/MW to the next without any gaps where they aren't supported, and to be able to remain on a combination for at least 1 year, preferably 2.
[[ https://www.php.net/supported-versions.php | PHP ]]
* PHP 7.1 support: Dec 2016 - Dec 2019
* PHP 7.2 support: Nov 2017 - Nov 2020
* PHP 7.3 support: Dec 2018 - Dec 2021
[[ https://ubuntu.com/about/release-cycle | Ubuntu ]]:
* Ubuntu 18.04 LTS: mid-2018 - mid-2023 (PHP 7.2)
* Ubuntu 20.04 LTS: mid-2020 - mid-2025 (PHP 7.3, maybe later)
[[ https://wiki.debian.org/DebianReleases | Debian ]]:
* Debian 9 Stretch: mid-2017 - 2022 (PHP 7.0)
* Debian 10 Buster: mid-2019 - 2022+ (PHP 7.3)
[[ https://www.mediawiki.org/wiki/Version_lifecycle | MediaWiki ]]
* MediaWiki 1.31: June 2018 - June 2021 (PHP 7.0+)
* MediaWiki 1.32: Jan 2019 - Jan 2020 (PHP 7.0+)
* MediaWiki 1.33: June 2019 - June 2020 (PHP …)
* MediaWiki 1.34: November 2019 - November 2020 (PHP …)
* MediaWiki 1.35 LTS: June 2020 - June 2023 (PHP …)
Now, let's see if we could require PHP 7.2 for MW 1.33+ and uphold the three assumptions.
The only combination we need to look at is Debian 9 Stretch with PHP 7.0. On that platform, we support MW 1.31 started June 2018, and supported until June 2021 (that's 3 years even). By then, there will be Debian 10 and several MediaWiki releases that support for PHP 7.2, which gives them a clear upgrade path without any gaps where they aren't supported.
It does mean however, that we don't support users who remain on a single Linux LTS for more than 3 years, but I don't see why we would need to support that. The above proposal means they can remain idle for 2 years, and then have the third year to upgrade.