Yahoo is definitely handling this badly: they should have offered an export tool, given a longer window, and been clearer in their messaging. But the claim that they didn't notify people (and just put the information up in a place no one would see) is wrong.
I'll add that some group owners/moderators have not even been able to log into their groups. Yahoo was doing something with needing a yahoo email to get in as admin, even when a non-yahoo email was associated with the account. Those people that lost access to their yahoo email account have been totally SOL.
When the G+ shutdown was announced (see earlier comment, I was one of the "Plexodus" organisers), the whole question of what to do about Communities migration hadn't even occurred to me. It wasn't until a community moderator happened to mention that they had a group with 400,000 subscribers and no idea at all to even begin that I started looking into this.
The first result was to create at least a framework for organising and planning: https://social.antefriguserat.de/index.php/Migrating_Google%...
It's a really basic thing. I've found no evidence that any such guide has ever existed online. I make no claims that what I came up with was the best, or even effective. It does seem to have been the first. Win, me.
We created a "short FAQ", formatted for posting to G+, which we posted again and again and again and again and again, to as many communities and spaces as we could. Because when you're trying to reach a mass audience (10ks to millions of users, 10ks to 8.1 million communities), mass repetition is necessary.
https://social.antefriguserat.de/index.php/G%2BMM_Short_FAQ
It's hard enough to motivate people to turn up at one place once every couple of years to mark a few spots on paper, when they all know that this happens regularly -- election turnouts tend around 30-40%. Getting mass compliance on a complicated, unexpected, and technical process (with social and legal entanglements) is far harder.
Many groups glide by on minimum maintenance for years. Whatever initial technical competence was required to set up the group be long-lost. At best, moderation skills are known (and even that is a long stretch). Migration is an entirely different skillset, one rarely exercised, and, as noted above, extraordinarily poorly supported.
TL;DR: your expectations are entirely unreasonable.
For every early adopter pissed off there will be a hundred or more people that never make it to your new and shiny project. So piss off enough early adopters and whatever you launch will be a dud.
Either Google have done the maths and determined they no longer rely on early adopters ...
... or they haven't, and are badly deluded.
Given the response I've noted to many Google announcements since ~2013 or so, I'm suspecting the latter. It's pretty strongly hostile.