The red-box note that replaced the instruction page still says "Yahoo has made a change to their login system...". That would seem to be out of date for the current outage.
From what you've said though it sounds like it might be sufficient to disable just the Transfer Members checkbox in the transfer forms.
True and true.
Based on comments on Y!GMF and GMF@ there are some groups who were on the cusp of transferring who'd be more than willing to take on inviting/adding their member addresses themselves. If the transfer mechanism could be used to get the members' content that would still be a big win for them.
The biggest support issue I can foresee (apart from the "what do I do about members" question) would be a need for post-transfer re-connection of members who joined later (by invite or otherwise) with their content transferred sooner. Maybe something operating during the join flow could look for that situation, but only if the information (original owner email address) is preserved somehow during the import.
GMF@ can try to help with the questions part, but not so much the re-connection part.
Or maybe going forward what's left of it needs to be a feature that requires an advance purchase of a year of Premium, as importing more than 100K messages does now.