Before people read too much into this. The Yamagata line was already in the decline and mutated little even from before the pandemic. Locking people or the world for extended periods of over a year or social distancing would not have the same impact as others have mentioned. Not worth the price, plus, we would not want for it to come back with a vengeance, until we are certain that it is eradicated. Since it seem that we would have to stop travel for it to make a more serious dent.

First, 90% that is not fully irradicated. Or at the very least there is no guarantee of such claim. So to claim this is in this case is not correct.

https://www.nature.com/articles/s41579-021-00642-4

"The stark reduction in global prevalence of B/Yamagata viruses compared with the other lineages may indicate an inherent vulnerability of this lineage. Indeed, B/Yamagata viruses have a lower effective reproductive number than B/Victoria viruses, and B/Yamagata epidemics have a slower initial growth phase with shorter transmission chains than B/Victoria epidemics5. This may make B/Yamagata more vulnerable to breakdowns in onward transmission, especially in the context of social distancing and movement restrictions. Furthermore, although multiple B/Yamagata clades can co-circulate for extended periods, previously long-lived clades went extinct6,7. Although the precise factors that drive B/Yamagata clades into extinction are unknown, the frequency of such extinctions suggests an intrinsic volatility in global B/Yamagata circulation. Lastly, as B/Victoria prevalence has been increasing since 2019, B/Yamagata may have already been at a low prevalence cycle at the beginning of the pandemic.

The global circulation patterns of influenza viruses have been associated with age-specific patterns of infection, whereby infection of adults, who are more likely to travel than children, correlates with greater global spread8. School-aged children and adolescents are more frequently infected with B/Victoria viruses, whereas adults over the age of 25 years are more frequently infected with B/Yamagata viruses5,7. Consistently, in 2008–2019, B/Yamagata viruses demonstrated greater global movement than B/Victoria viruses7. The reduction in global mobility due to the COVID-19 pandemic may have contributed to low distribution and seeding of B/Yamagata. Overall, the temporal and intrinsic epidemiology of B/Yamagata viruses may have rendered them particularly vulnerable to this effect.

From 2017, circulating B/Yamagata viruses have all been from a single lineage (clade 3A), whereas several lineages existed before. Although this clade has diversified at the genomic level, and even acquired amino acid substitution in haemagglutinin, these have not resulted in significant antigenic changes to necessitate updating the B/Yamagata vaccine component since 2015 (refs6,7). Therefore, although B/Yamagata viruses have continued to evolve genetically, they remain antigenically conserved in the haemagglutinin protein in recent years. Thus, the onset of the COVID-19 pandemic coinciding with a period of low incidence and low antigenic diversity, coupled with prolonged use of a well-matched influenza vaccine, may have enabled massive suppression of B/Yamagata during the pandemic. However, as sampling and sequencing is not comprehensive, it is difficult to distinguish with certainty between lack of detection and true extinction."