RemovePaywall (the add-on) is a launcher. When you click its icon, press its shortcut, or use its right-click menu, it opens https://removepaywall.co with the address of the page you chose, so the site can show that article in a reader view. It sends nothing else, makes no background requests, and keeps no browsing history.

When the add-on is installed or removed it opens a page on removepaywall.co, which the site counts. On the removal page you can optionally tap a reason; only that reason is recorded.

What removepaywall.co itself collects (request logs, aggregate statistics, cookies) is described in full at https://removepaywall.co/privacy