Yeah. Apple will reject your app if you have a NSFW toggle. It’s kinda weird, because Apple seems to only enforce the rules if they notice you break them. There are likely other iOS apps that have a NSFW toggle which they haven’t noticed (e.g. Voyager).
If your app includes user-generated content from a web-based service, it may display incidental mature “NSFW” content, provided that the content is hidden by default and only displayed when the user turns it on via your website.
However, there’s no reason Blorp can’t show this toggled when installed outside the App Store.