TL;DR unless the shitlib did a Mission Impossible style heist, all the registration data is just on someone's server, likely in a massive data center wherever they were paying to host the site from. It's probably encrypted (well, it should be) and effectively inaccessible to anyone without the proper keys to access it.
The domain name is just the domain name
When you type in a domain name your DNS resolves that name to an IP address
Without dragging this out overly much, it's like someone stole your phone number. Not your phone.
When people call your stolen number they expect that Illuminati answers. The thief could fake being you, if they were good enough and wanted to.
But they don't have your data. That resides on your phone. The server(s) that were being used to host hexbear, to leave the analogy behind.
There's currently nothing to worry about data theft side of things.
However, with minimum due respect, whoever managed to let the domain registration expire is a top tier... dingus. I honestly don't even know how they let that happen. 🤷♂️