The user “sochi-russia”, a RuNet forum member since 2012 highlighted on December 15th that they were seeing a new SERP feature when roaming Yandex.
This new yellow feature was later confirmed as a test by Dmitry Gromov (Yandex employee). It has been described as a “regular” test to see how Yandex can improve both user experience, and it’s paid advertising functionality.