rec.music.folk (and uk.music.folk) - which are not email groups and are nothing to do wifh Google - still exist and I still monitor them - and v e r y occasionally post. But they're mostly derelict at the moment. They just need people to start posting again.

On Windows you can use a dedicated usenet client, such as Free Agent, or Mozilla Thunderbird, and a free usenet account from eternal-september. On iOS there is a client called NewsTap - I've not used it. There are still active usenet grouos, but not for folk music. But when Mudcat dies, usenet will still be there.