i dont know if there are any good alternatives. surprised its held on so long. but cost models elsewhere eliminate most alternatives, people are just greedy. though a forum could serve the same needs it may fail for the same cost reasons similar to this forum where we have very restrictive image upload terms. perhaps github if the group generates a lot of files and needs space and older file versions but not sure on the discussion side. a forum would serve the same needs if it didnt require huge space needs (that might still be served by google drive/dropbox/github/etc)
I don't know about or speak for what can or cant be done here but hypothetically lets consider:
currently IIS has a forum containing main areas such as General Astronomy, Equipment and IceTrade Classifieds with broad sub sections underneath.
We could have say a "BASS Group" main area containing subsections like
- About The Group
Where its outlined what the groups is about, its history and members, links to any existing yahoo stuff etc.
- Files
Where the primary program current version(s) are directly downloadable along with change notes, source code etc. Also a place for example datasets to be available to learn with. A place for users to upload files too.
- Discussion
Where chatter happens on whatever aspects of the group and its area of interest can be
- Bug reports
Where bugs can be reported along with relevant data and steps for replication etc.
Something like this should cover most groups/projects need without adding too many needless subgroups to clutter and confuse. In particular if a new forum was created each area could be created using a thought out template of subsections. In a forum each subsection can have a bunch of "sticky" and "locked" threads at the top for vital information and guidelines. the people can post their threads in there. Just takes a bit of familiarity with the existing groups to determine the high level grouping needs of the information and the sub groupings. then you need moderators to keep everything clean of spam (external) and abuse (internal). Cost is mostly about required storage to get running, but then there's things like uptime of the server(s) backup/restore/archiving regimes. software and hardware administration, repairs, upgrades. A forum is dead easy for anyone to setup on a home computer in under an hour, but a forum thats online 24/7 and open to users globally is a whole other ballgame and costs are vastly different from your home pc.
plus apart from all that, WHO is doing this to get it running and maintained? will they do it indefinitely free of charge for the love of it? not likely? it takes a lot more time and effort than people think so expect it to require money to get the ball rolling or continue rolling at some point.
RIGHT NOW, I highly suggest the groups you care about and thinking of migrating elsewhere grab yourself HTTrack (free) to download as much of those yahoo groups that still exist. Its a web ripper, it follows the links in the html code and you may have to tweek the type of links it'll follow and how deep it'll go. But this will get whats still there today and save it with changed localised links so you can navigate it all on your computer. Its not a form that will let you transfer to another system though. yahoo could probably provide that if you asked and gave them money. but right now rip the site yourself so the files and content are saved. later of you might need to sort through the content and put into another form to put into another system, but at least you'll have it. get it safely first then deal with it later.
AS WELL as running httrack I dont know if yahoo groups has a flat or printable thread view at all, but if theres something like that to see the entirity of a single discussion thread use that to print that thread to a pdf to presrve the discussion and history. in a new system you might manually start a thread with the same title and post the pdf as the first post so users can read and continue the discussion . and yes do this for every thread is ideal. and yes very very very time consuming. but nothing compared to rebuilding new threads/posts from old conversations. its also not something that has to be fully done first, get the most active/important threads done and published in a new system for users to use. then rebuild the others and publish those as time allows. there is a ton of manual work involved. way more than a ton actually. may take a year to get the old threads on a new system. its not a simple copy and paste. if you can get the groups from yahoo in a standardised format then you may be able to import into another database but thats never a simple task either, there will be problems and cleanup and manually checking cleaning. enjoy!