Other LSMs are slowly switching to syscalls too, and while I in principle like (and have abused) the whole "everything is a file" principle, most security mechanisms really should be done via special-purpose syscalls. Way too many footguns with filesystem-based APIs. Also, you wouldn't be able to use Landlock to restrict filesystem access based on dirfds with a filesystem-based API.
The questions you have about seccomp depend on the rules. Well-written filters would return -ENOSYS in that case, so it would look to the program as though the syscall is unsupported.
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Other LSMs are slowly switching to syscalls too, and while I in principle like (and have abused) the whole "everything is a file" principle, most security mechanisms really should be done via special-purpose syscalls. Way too many footguns with filesystem-based APIs. Also, you wouldn't be able to use Landlock to restrict filesystem access based on dirfds with a filesystem-based API.
The questions you have about seccomp depend on the rules. Well-written filters would return -ENOSYS in that case, so it would look to the program as though the syscall is unsupported.