Any proof of that? I’ve always found Yandex to be substandard but never for reasons of censorship.

Yep, plenty, and tbf it's entirely logical. The people working at Yandex do not want find themselves dying of a nerve agent or polonium, which is a real and acute danger for people who displease the ruling regime.

https://hal.science/hal-03217497

https://euvsdisinfo.eu/yandex-from-tech-innovation-to-inform...

https://pmc.ncbi.nlm.nih.gov/articles/PMC10130930/

https://misinforeview.hks.harvard.edu/article/a-story-of-non...