Gab was "banned" in that the vast majority of instances blocked it and blocked any other instance that didn't do the same.
If that's not censorship, I don't know what is. You can have your sandbox but you can't play with others unless you accept their censorship rules and politics.
https://news.ycombinator.com/item?id=28495086
http://ceur-ws.org/Vol-2737/FP_6.pdf
There was a huge uproar about it a couple of years back.
EDIT:
https://www.reddit.com/r/Mastodon/comments/jkiw1y/can_someon...
Gab was "banned" in that the vast majority of instances blocked it and blocked any other instance that didn't do the same.
If that's not censorship, I don't know what is. You can have your sandbox but you can't play with others unless you accept their censorship rules and politics.