https://www.mediawiki.org/wiki/Compatibility#Mobile
Where browser usage is over 5% a modern experience (Grade A) is supported.
Grade A browser list on MobileFrontend is defined in .browserlistsrc file.
Looking at https://analytics.wikimedia.org/dashboards/browsers/#mobile-site-by-os/os-family-and-major-hierarchical-view only 12 and 13 are over 5%; 11 is 2.2%, 10 is 1.3%, 9 is 0.87%
https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/4f8ed0f/.browserslistrc#L15 says ios 6.0
Similarly, only Android 6.0 is > 5%, with Android 5.0 at 3.1% and 4.0/4.1 absent completely
https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/4f8ed0f/.browserslistrc#L17 says android 4.1
The ios version in .browserslistrc also doesn't agree with https://www.mediawiki.org/wiki/Compatibility#Desktop (6.1 vs 6.0)