Believe it or not there have been quite a few threads about this:
Why Russians do not smile (2002) - https://news.ycombinator.com/item?id=27317859 - May 2021 (481 comments)
What a Russian Smile Means - https://news.ycombinator.com/item?id=17445108 - July 2018 (67 comments)
What a Russian Smile Means - https://news.ycombinator.com/item?id=17381975 - June 2018 (1 comment)
What a Russian Smile Means - https://news.ycombinator.com/item?id=17376212 - June 2018 (1 comment)
Do Russians smile at each other? - https://news.ycombinator.com/item?id=7491944 - March 2014 (1 comment)
Why do Russians smile so little (and Americans so much?) - https://news.ycombinator.com/item?id=2375633 - March 2011 (105 comments)