From the point of view of an app developer, it might not make sense to filters peers but services (ports) instead, and filtering peers without their names would not be ideal (the kernel doesn't know about DNS, only IPs). Anyway, this feature might come one day if someone want to work on it, but we follow well-tested incremental development.
Netfiler is a privileged network feature that allows to do almost anything with the network, which makes it unsuitable for (app/unprivileged) sandboxing.
A rough description of upcoming network restriction features in Landlock and how they map to the BSD socket API is in the talk at https://youtu.be/K2onopkMhuM?start=2025 starting around 33:45
I really hope we can get back to these features soon :) I think these would be very useful.
...of course it is common enough that it would make sense to abstract over the different protocols that have more or less the same concept of ports.