It's being done using devices that only run software approved by Apple, and Apple already uses that power to dictate what communications are allowed. By their mandate, apps like Discord and Telegram are forced to hard-block users on iOS from joining or viewing servers and communities with adult content, and for some communities with a more, ah, relaxed attitude to sex this fundamentally affects how they communicate with their friends. It doesn't matter what communications software they use because the mandate comes from Apple and every single competing option is required to impose the same restrictions. Arguably this restriction is going to shape how our culture thinks in the long term too.

That is like saying Microsoft Windows is not a monopoly in the 90s because they have a choice to choose Mac or Linux.