The Less variable @font-family-sans has been marked for deprecation since v0.13.0 (2018-12-05) in WikimediaUI Base: source.
Repositories using this variable: https://codesearch.wmflabs.org/search/?q=font-family-sans%5B%5E-%5D&i=nope&files=&repos=
As of 2020-02-20 the only user in tracked repositories that does not redeclare this variable is Wikibase:
https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/ffd6ef8ce371d39b1437550e5894393e1f4c231e/client/data-bridge/src/presentation/components/EventEmittingButton.vue#L154
And wikibase-termbox: https://codesearch.wmflabs.org/search/?q=font-family-sans%5B%5E-%5D&i=nope&files=&repos=wikibase-termbox