A member of ArchiveTeam did a fairly comprehensive crawl of the messages of public groups between 2015 and 2018. And there had been discussion about how to archive Yahoo Groups for a few years prior to that as well.

The reality is that in order to archive a site as large as Yahoo Groups, you need a critical mass of people willing to commit time and energy to the project. And people are more likely to commit to an "urgent" project, where there is a set deadline.