Ha! Last year I did some preliminary research, and all contemporary sources I've found agreed that last public search engine stopped supporting meta keywords around 2018, IIUC. So good catch about Yandex, thanks for the headsup! Funny, how things get complicated :) This meta had very turbulent history in 2010s and refuses to die, apparently.
Mention of usage by Yandex: https://yandex.com/support/webmaster/en/controlling-robot/me...