Ask HN: More background on these Brave browser findings?

Interesting thread on the fediverse about Brave browser, but lacking more background and sources. This toot [0] and this msg:

"first, they sold exceptions to privacy tracking to Facebook and Twitter, now it comes out they've got a header override that allows them to set header exceptions on demand."

[0] https://glitch.social/@tessaracht/101572578668968885