Mastodon started as a platform with a blocklist. Eugen didn't like some things, so he blocked them. Mastodon's developer volunteers were notably acidic when asked about making sites work better together, and Eugen nearly had a cow when one of the big Japanese drawn porn sites opened a mastodon instance. I had an account on the original mastodon.social system, you couldn't see things from certain other *social sites because they were blocked for "reasons," according to Eugen. One of those was the sealion.club site I was using at the time, it was a controversial site because it came out of the hysteria of gamergate, but rapidly settled into a "As long as it's not illegal, you can say it" site.