1.34 is the trailing legacy release and requires 7.2.9 or newer. 1.31 LTS, supported into 2021, requires 7.0 or later.
In releases since 1.27 introduced PHP 7 support, the backward version compatibility has been tightening up more aggressively. PHP 7 brought a ton of performance gains that MW had previously relied on HHVM for, which gave them a lot of reasons to shed PHP 5 and start turning more forward-looking.
Wikipedia is running php 7.2 https://en.wikipedia.org/wiki/Special:Version so no. That said, new features do get used when they make sense, so we are definitely not maintaining back compat with ancient versions of php.
(This includes a couple usages of traits, traits is just not something super common in mediawiki, but there are some cases where they are used)
Wikimedia is only provisionally on 7.2 until they finish migrating ahead of the next Mediawiki LTS release. There are a bunch of points where they talk about the benefits of requiring 7.3 here: https://phabricator.wikimedia.org/T257879
For a project as widely used as Mediawiki, it completely makes sense to iterate slowly as PHP evolves. New features are a wonderful opportunity in many cases, but without a full understanding of how they may affect a very large code, caution is well warranted.
