Hey, I'm actually the creator of the plugin. I'm amazed to see a picture of it on HN. Thanks for using it and let me know if you have any questions. If anyone else wants to use it, it's called Plain Text Offenders Alert and available in the chrome web store.
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.