The owner of spcr.netlify.com made the users' test results, which this script relies on, deliberately inaccessible. The best you can do is install this basic browser add-on, which uses what little information they still can get.
Remember, kids: never rely on strangers.
This script will show you a game's SPC rating(s) on the following websites, unless the game already has an official Linux version. You can click on it to open the game's report on https://spcr.netlify.com/ with the full Proton testing results.
A small, non-intrusive badge will be added near the "buy" button on each game's shop page with the most frequent rating next to it, or "N/A" if there are no test results yet.
An additional row to each game's info table will be added, showing a summary of the given ratings and their count.
I've only tested this on Firefox with Greasemonkey 4, so if you're deviating from that and it doesn't work at all, submit an issue up top. It should also work with Tampermonkey since v0.3 though.