May be I'm biased but a distinction between base OS and 3rd party software is a simple concept to me, also it is not unique to BSD. E. g. in Windows there were different menu sections to install/uninstall optional Windows components and to uninstall 3rd party software.
If aynthing Linux is an outlier in that core OS components and 3rd party apps both come in similar packages which makes it hard to distinguish which is a part of the base OS and which and optional 3rd party software. In FreeBSD I know that I can delete all packages and still have a system remotely accessible via SSH (with rare exception a normal user will not come across). In Linux to tell which package can be uninstalled without breaking the system one have to be an expert. Can I delete iproute package if I don't use ip command, can I remove NetworkManager, is systemd-container optional? Where the OS stops and 3rd party apps start?
