This could change at any time, because extensions are automatically updated (unless you change the setting), and the permissions when it's installed allow it to

This could change at any time, because extensions are automatically updated (unless you change the setting), and the permissions when it's installed allow it to, so it could start sending the text in the active text area, for example. There's also no way for them to prove that they don't store the text that's sent to them. Chrome used to always show a list of requested permissions when installing an extension, but it doesn't anymore, unless it's asking for special permissions. Extensions can do a lot without special permissions.