From the chrome webstore page:

>Disclaimer 1: Plain Text Offenders Alert is not associated with plaintextoffenders.com, it uses their publicly available database.

plaintextoffenders.com itself works on reports. My extension uses an api (https://8ack.de/plato/info/). The extension regularly caches the list of offenders and checks your current url against the list locally, so neither I nor the api creators can see your browsing history.