It's November 2019 now, Yahoo's removing content around 14 December. A number of Yahoo Groups are in the process of moving to groups.io.
A move costs in the region of $220 dollars to complete the migration and for the first year. It's possible to change to a cheaper or free plan thereafter, depending on how much space is needed for content and so forth.
Dec 1 is the cutoff date for "guaranteeing a successful migration", meaning that to be sure that all messages and so on are transferred across it would be best not to attempt this after Dec 1.
I've taken an offline backup of this group but it's a backup of last resort and there are limitations with regards to the message (email) data. It is possible to sort the messages and copy the content into PDF archives but the formatting won't look wonderful.
A professional migration to a site such as groups.io will cost $220 to accomplish, thereafter costs should be zero or minimal.
Alternatively one can create another AN1x group at groups.io and forego the professional migration and restore content manually. It's probably simple enough to do for photos and files but there doesn't appear to be a way to restore old messages without paying.
A migration to another host should be considered and there isn't much time left to make a decision.