Thanks for asking. I was using incorrect terminology. It’s the Influenza B/Yamagata lineage which has probably gone extinct. According to this paper in The Lancet : https://www.thelancet.com/journals/lanmic/article/PIIS2666-5247(24)00066-1/fulltext
The B/Victoria lineage persists. B/Yamagata has not been confirmed since March 2020 according to this paper : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9524051/
This paper from Nature goes into it a bit more, in particular wrt implications for vaccines: https://www.nature.com/articles/s41579-021-00642-4
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