These extensions work first by looking at the contents of the page you’re on to detect a paywall, and then make modifications to the page that remove the paywall. There’s no way for the browser-creators to guarantee that the extension isn’t also silently adding a hidden element that captures everything that you type into that website, in addition to the paywal removal, so they’re basically trying to warn you such a thing could happen.

And that is a genuine risk from every extension in the addons store, but I would say that risk is potentially even higher with a piracy extension installed from a github relese. (Not this one in particular per se, which I have no opinion about, just in general.) If it makes you uncomfortable, a reasonable compromise could be to create a new browser profile for use only with this extension, or maybe even use a different browser entirely than your daily driver.