It was originally developed by Kiwi Farms when they were running their own Mastodon instance.
They built this tool because they were being massively defederated (for obvious reasons) but eventually gave up and closed their Mastodon instance.
Since then, other instances apparently not related to Kiwi Farms (but usually still that kind of "free speech" ones) have reinstantiated the service.
It also has a slur immediately on the page you linked