Well, Instagram never claimed to be a free speech network. Also Instagram is covered with content that sexually exploits children, and Telegram links to CSAM were pretty common.

Given that Telegram has very publicly failed at that kind of moderation, it is pretty defensible to block it. If only they would do something about the weirdo Instagram mommies that flood the network with pictures of their preteen daughters in swimsuit and leotards... It's gross what some people do for anything resembling Internet clout.