Suppressing substack when substack is trying to advertise for a twitter competitor is just common sense. There's no need for twitter to advertise for a competitor. That isn't censorship, thats just normal business. If substack was depending on twitter for free data and free advertising they were the ones who were weak and nervous.
That being said, maybe the important things will start being said in private spaces and only end up in "public squares" via screenshot ten or fifteen minutes later.
E.g. an author writes content, people who are not affiliated with substack try to like/share.
That is not advertising, and this from the apparent free speech absolutist Elon.
They can block whomever they want and I can say it highlights how fragile they view their own business position as.
Twitter is within their right to cut off access to their competitors, future or present.