Since 2019 Yandex and the FSB (Federal Security Bureau of Russia, ex-KGB) had reached an agreement where the company would provide the required data without handing over the encryption keys.[0]
Learn more about Russian war crimes & ongoing invasion of Ukraine: https://en.wikipedia.org/wiki/2022_Russian_invasion_of_Ukrai...
N.B. I'm Ukrainian living in Ukraine. Here is my statement for HN:[1]
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[0] https://yandex.com/company/press_center/press_releases/2022/...
Edit: This report appears to bear out that the sanctions forced him to resign pretty-much-directly, as the company does business in Europe, and the sanctions prevent him from doing business in Europe: https://www.datacenterdynamics.com/en/news/yandex-deputy-ceo...
You mention oligarchs - how can it be ruled out that Yandex is directly or indirectly, fully or partly owned, and even directed, by them?