No, while free newsgroups are common, there aren’t any free full-blown Usenet providers. The costs involved in running servers, maintaining software, and running the site and systems wouldn’t make it viable. There are plenty of free trials and some that give free Usenet access with no credit card.
We are talking about a trial for a Usenet provider, not a newsgroup free trial. Access to newsgroups comes with the provider.
You can also get free Usenet search engines or a free Usenet indexer, but to get the full experience you will usually have to pay a subscription or one-off fee.
If you are particularly tech-savvy and have access to web servers, you could in theory host and run free newsgroup servers. This itself isn’t really ‘free’ because you’re paying for the servers.
While Usenet is open to anyone and technically you can enter a public newsgroup or private newsgroups free of charge, there are a lot of hoops to jump through that do cost money.