1、Does Captive Portal documentation for IOS exist for developers? If so, please provide a link.
This is seemingly enterprise networking, and — outside of NEHotspotHelper — not particularly development-related.
Links related to this topic, depending on where you're headed here:
https://developer.apple.com/news/?id=q78sq5rv
https://tools.ietf.org/html/draft-ietf-capport-rfc7710bis
https://tools.ietf.org/html/draft-ietf-capport-api
https://support.apple.com/en-us/HT204497
https://developer.apple.com/documentation/NetworkExtension/NEHotspotHelper
https://www.securew2.com/blog/apple-captive-network-assistant-understanding-captive-portals-on-ios-devices
https://community.cisco.com/t5/security-knowledge-base/dual-ssid-byod-with-apple-captive-network-assistant-cna-browser/ta-p/3642663
There are other discussions here in the developer forms, as well.