I finally got my official YahooGroups download. It took almost exactly two weeks, and I'm only in 7 groups. It's a zip full of zips: messages, files, links.
The messages are in numbered MBOX files. On the upside, they import to Thunderbird just as readily as the Chrome addon results. On the downside, each MBOX file has several dozen messages that are missing information. Sometimes date/time/sender/subject - those tend to have a single word as the sender and no subject and timestamped when I opened the imported folder. Looks like MadLibs in the Sender list. Sometimes the body of the message is blank. They're kind of a mess. I don't know how much of that is the import into Thunderbird, and how much is defect in the MBOX file, though I didn't see this kind of mess from the MBOX generated by the Chrome addon.
The Photos section is missing altogether. Apparently Yahoo is having Issues with the Photos section, and is unlikely to fix it since they're planning to purge it anyway. Import to Groups.io may not get the Photos section because of Yahoo's unfixed Issues.
The images that were in the Files section are there, but may or may not have their file extension. Some of them have the extension as part of the file name and are just missing the dot that makes it an extension. This goes for other files in the files section, as well. Files need to be checked against the archive to make sure they get their correct extensions so they're still usable.
Upshot is, for accurate archiving, the manual download of files and the Chrome addon or PGOffline are necessary. They can be uploaded to filesharing options like Dropbox or GoogleDrive or Microsoft OneDrive etc, or hosted at ISPs. Squidge.org has the advantage of being a listserver, so we could start over, and it provides filehosting and website hosting, so we could upload the old archives in addition to having the new archives, all in the same place. Of all the options I've found, this looks like the best to me.
no subject
The messages are in numbered MBOX files. On the upside, they import to Thunderbird just as readily as the Chrome addon results. On the downside, each MBOX file has several dozen messages that are missing information. Sometimes date/time/sender/subject - those tend to have a single word as the sender and no subject and timestamped when I opened the imported folder. Looks like MadLibs in the Sender list. Sometimes the body of the message is blank. They're kind of a mess. I don't know how much of that is the import into Thunderbird, and how much is defect in the MBOX file, though I didn't see this kind of mess from the MBOX generated by the Chrome addon.
The Photos section is missing altogether. Apparently Yahoo is having Issues with the Photos section, and is unlikely to fix it since they're planning to purge it anyway. Import to Groups.io may not get the Photos section because of Yahoo's unfixed Issues.
The images that were in the Files section are there, but may or may not have their file extension. Some of them have the extension as part of the file name and are just missing the dot that makes it an extension. This goes for other files in the files section, as well. Files need to be checked against the archive to make sure they get their correct extensions so they're still usable.
Upshot is, for accurate archiving, the manual download of files and the Chrome addon or PGOffline are necessary. They can be uploaded to filesharing options like Dropbox or GoogleDrive or Microsoft OneDrive etc, or hosted at ISPs. Squidge.org has the advantage of being a listserver, so we could start over, and it provides filehosting and website hosting, so we could upload the old archives in addition to having the new archives, all in the same place. Of all the options I've found, this looks like the best to me.