According to Wikitech, iOS9 is being taken out of commission by the Internet (Let's Encrypt root expiry). Should the compatibility table be updated? There may be other benefits according to Caniuse.
Topic on Talk:Compatibility
This page documents compatibility of the MediaWiki core software, not the Wikimedia Foundation deployment and infrastructure as used for Wikipedia. There are hunderds of software packages and services that make up the Foundation's infrastructure. The subset of all those make up what Wikipedia offers in total, and what it requires at minimum. That is not documented on this page. For the specific aspect of HTTPS requirements, that is documented on Wikitech indeed.
MediaWiki can be used with any web server and (if using HTTPS) any certificate authoritity. It does not have to be Let's Encrypt.
Having said that, there are plans to remove iOS 9 support for MediaWiki's additive "Modern" JS-based features over the near year, as part of T178356. The exact iOS version to require for our subset of ES2015 features is still to be determined, but there is consensus that it will include at least "Promise" and "Promise-finally" which would require iOS 11.