Steve wrote, "So perhaps it's time to say goodbye to everyone left on YahooGroups and warn them that there is nowhere to flee to ..."
What? Why do you say that?
Isn't there Groups.io?
Groups.io IS free -- it has a free tier -- and the free level gives you the same functionality (and more) that Yahoo!Groups has today. Which is, it's an email message reflector, with no permanent storage for Files and Photos. For new groups, you only need to pay if you need separate Files and Photos sections, that is, having them separate from Attachments (where they are still free). What's more, Groups.io gives you something that Yahoo!Groups does not have: permanent storage of messages, and permanent storage of Attached Files and Photos. This makes Groups.io BETTER than Yahoo!Groups is today, for the same price!
And if you created your Groups.io group before August 2020, then you have the separate Files and Photos sections for free too, which makes it functionally equivalent to the features of Yahoo!Groups in its heyday, for free.
If you didn't create your Groups.io before August 2020, then you weren't paying attention when Yahoo pulled the rug out from under you in 2019 and erased all your archived messages and files. If that didn't matter to you then, then the FREE tier from Groups.io should suit you just fine!
To me, that IS an alternative. It's a very good alternative. It is a place to flee to.
I manage a few groups on Groups.io and I'm happy enough with it. There are some annoyances, for sure, from the Moderator's viewpoint, but regular users for the most part don't see that.