This class seems unused and should probably be removed? If there are any discussions why the class is still around, here could be a good place to discuss and know the way forward.
A patch already exist and it will be linked to this ticket.
# Acceptance criteria
[x] Drop SpecialMobileMenu class
[] Drop the MobileFrontend MobileMenu aliases (https://gerrit.wikimedia.org/g/mediawiki/extensions/MobileFrontend/+/32600eea86f105d53227487cde1294b143bbede6/MobileFrontend.alias.php#18)
[x] Drop codepath in onSpecialPageBeforeExecute https://gerrit.wikimedia.org/r/c/mediawiki/skins/MinervaNeue/+/615824
[x] Drop isSpecialMobileMenuPage property on MinervaTemplate https://gerrit.wikimedia.org/g/mediawiki/skins/MinervaNeue/+/b5c20d2d64f43bcc1c3f4e1a0242e570ee40e673/includes/Skins/MinervaTemplate.php#45
[x] Drop the unused .navigation-full-screen class https://codesearch.wmcloud.org/search/?q=navigation-full-screen&i=nope&files=&repos=
[x] Drop the unused MobileMenu skin style https://gerrit.wikimedia.org/g/mediawiki/skins/MinervaNeue/+/b5c20d2d64f43bcc1c3f4e1a0242e570ee40e673/skinStyles/mobile.special.mobilemenu.styles/minerva.less