A lot of the online services that were launched back in the 1990s fell out of favour with users over time, however many of them were still running. Yahoo!, which has changed hands multiple times over the years, offered a mailing list and group service called "Yahoo Groups". I'm not sure how many active users would still be on those groups, but while you couldn't create new ones for the last while the service was still running.
From tomorrow it'll be gone:
Announcement: End of Yahoo Groups We’re shutting down the Yahoo Groups website on December 15, 2020 and members will no longer be able to send or receive emails from Yahoo Groups. Yahoo Mail features will continue to function as expected and there will be no changes to your Yahoo Mail account, emails, photos or other inbox content. There will also be no changes to other Yahoo properties or services. You can find more information about the Yahoo Groups shutdown and alternative service options on this help page.
They're suggesting that people switch their groups over to a number of different services, both free and paid.
I've got fond memories of some of the old Yahoo Groups where some mailing lists I was on used to live. Most of them either died off or moved elsewhere years ago, but there were still a few "hold outs".
Most of the services I used back in the 1990s have gone. Some of course haven't, though the 2020 version looks very very different!