From an excellent suggestion from Duane, I've added the following to the top of the Yahoo Transfer page:
Due to the pending discontinuation of many parts of Yahoo Groups, we ask that all transfer requests be given the go ahead no later than December 1, 2019. We will not be able to guarantee any transfers submitted after that. Also note that it appears that most Photos sections are already off-line and not transferrable.
Please let me know if there are other places this should be added to, or if you have any suggestions for improvements.