A previous known tool to batch download Google Groups messages, https://github.com/icy/google-group-crawler, does not work with the current (as of March 2023) Google Groups website.
What I needed was to download all attachments from a small number of conversations. So I used some JavaScript to automate downloading them from my browser. It works like this:
- Open the Google Groups app and insert the initialization code into the Console:
function sleep(ms) {
return new Promise(r => setTimeout(r, ms))
}