Who knows what that extension is doing behind the scenes. The cynic in me says the github page is only there so people who google 'tampermonkey source' or 'tampermonkey license' are tricked into thinking it's a FOSS/privacy-friendly choice, unless they take a closer look. The most recent commit is just for a github issue template. In reality all development done in the past 10 years has been closed-source. Only that's the source from 10 years ago. The project has a Github with source licensed under GPL. I'd been using it for years to run a script that bypasses Youtube age verification without having an account.
I always thought Tampermonkey was open-source, considering its popularity among hobbyists. People in a chat I'm in were discussing this issue and I was shocked. TLDR: Tampermonkey (a browser extension that allows you to run custom scripts on any site conveniently, without writing and publishing your own extension) has been closed-source for 10 years
