You should be able to deny/allow connections for any app, including 1st party apps.
This isn’t only an issue of trust. There are apps made to meter your connection (e.g. when you’re using your phone as a Personal Hotspot), where you’d like to see how much bw your 1st party apps are using, and have the option to block them.
Yes, but you won't have much visibility into anything encrypted (HTTPS). Which is everything. You'll just see opaque traffic going to certain IP addresses and you also won't know what process on the computer caused it. Doing this in the OS is way nicer.
Personally, I fixed this by getting an unlimited mobile data subscription, which was only €20/month here in The Netherlands. But I understand that most people can't get that.
What does seem weird to me, is that they didn't implement a low data mode into macOS, like they do with iOS.
You should be able to deny/allow connections for any app, including 1st party apps.
This isn’t only an issue of trust. There are apps made to meter your connection (e.g. when you’re using your phone as a Personal Hotspot), where you’d like to see how much bw your 1st party apps are using, and have the option to block them.