I think one difference in public access vs. other access is that in public access (for right or wrong) there are commercial incentives to give you misleading information in order to trick your device into behaving a desired way.
I agree that (yet another) API might not be making anything easier. Which is why I view RFC7710 + ICMP as a very simple solution, that builds on top of basic, existing, 'building blocks'. DHCP/RA giving a bit of network configuration (which it does today), and ICMP giving you feedback about the fate of your packets (something it does today). The NAS isn't doing anything different -- it is still deciding what gets through and what doesn't. The NAS is probably already sending ICMP for blocked traffic (or it isn't, leading to connection timeouts). Implementing the NAS portion is super simple (and can be implemented in may places in your network - iptables, separate rate limiter, etc).
I view this relatively SIMPLE protocol, which at its core is just telling you the 'truth' (or at least as close as you can get to it, subject to the same risks as ICMP in general) about what happened to your packets and why.