Some Yahoo Groups must be still active as mailing lists, or at least an archive for some niche communities, and they have irreplaceable, valuable information. I don't know other groups, but I know the Tektronix oscilloscope group (TekScopes) has been on Yahoo for a decade, and it's often the only source of information about vintage Tektronix oscilloscopes that dated back to the 1960s, some were former engineers with firsthand experience who can help to fix your scope or identify a replacement part, and the mailing list archive has a lot of lost knowledge that cannot be found elsewhere.
Well, fortunately TekScopes migrated to Groups.io in recent years, which is good. But just think about other groups...