This is because according to a post on Discord’s website, it appears that the company has decided to block iOS users from accessing NSFW servers. It also seems to have been done on purpose. According to the post, “Additionally, users on the iOS platform (including those aged 18+) will be blocked from joining and accessing NSFW servers. Only NSFW server owners and moderators will be allowed to view and manage these servers on iOS. All other users will still be able to join and access NSFW servers on the desktop and web versions of Discord.”
It is unclear why Discord has suddenly banned iOS users from accessing NSFW content, but it has been suggested that this could be due to Apple’s guidelines which says that apps with user-generated content or services that contain NSFW content could be removed. The company has not specified the reason why, but this is a possibility.
It is possible that Discord could create an option on its desktop app or website that lets users toggle on NSFW content, which would be in line with Apple’s guidelines, but for now iOS users are out of luck.