That was already discussed before, it makes no sense as News provides more features than RSS/Atom.
Thunderbird does not know about read/unread/starred messages of the News app and i have to read them twice.
Exactly that would not work with exposing an RSS feed.
The only solution so that you can use Thunderbird with news would be if there was integration in Thunderbird or an add-on that does that and that implements the news api like any other client does.
2018: [Feature Request] Support synchronising to Thunderbird's RSS reader · Issue #315 · nextcloud/news · GitHub
2020: [Feature Request] Support synchronising to Thunderbird's RSS reader · Issue #718 · nextcloud/news · GitHub
Also see Link RSS reader to Nextcloud News | Thunderbird Support Forum | Mozilla Support
But I think nobody there understood what news actually is, we are not a provider of an rss feed, rss doesn’t support “mark as read”, “star” and so on, thats why news has an API which is not the best at the moment but working for many clients.
As an alternative we have a list of supported clients maybe one of them fits your need. GitHub - nextcloud/news: 📰 RSS/Atom feed reader