.box-shadow() LESS mixin is now obsolete as box-shadow is supported without any vendor prefix fallbacks.
To support the goal in T277819, we need to update our extensions.
Popups is the only extension that web maintains that uses the deprecated mixin
https://gerrit.wikimedia.org/g/mediawiki/extensions/Popups/+/2f06a49a6ce63f66a7ba32162b77a968a3b38de6/src/ui/templates/popup/popup.less#23
We kinda support GuidedTour as we have @phuedx
https://gerrit.wikimedia.org/g/mediawiki/extensions/GuidedTour/+/433d6975a030314754113d3881b9375f2e52ada8/modules/ext.guidedTour.less#20
Gettingstarted can be ignored because of https://phabricator.wikimedia.org/T235752