Part of the problem is that Linux supports a much larger set of architectures and boot loader than OpenBSD. So trying to use a seed file which is read by the bootloader at boot time is hard.
Could systemd choose to read a seed file after it mounts the root file system? Sure, but then it's on systemd to believe that the seed file is always secure, even on IOT devices where the seed file might be imaged onto millions of devices with an identical value out of the box. Using a seed file means you also have to trust how the OS is installed; security is a holistic property involving the entire system's design and implementation.
Ultimately, it's not going to be up to kernel or systemd to tell users and system administrators what they should or shouldn't trust. If you trust RDRAND, and you're on x86, you can enable the config option or provide the boot command line flag, and you're all set. But I'm not going to tell you one way or another whether or not you should trust RDRAND. And even if I did, you could just reject my advice, either way. As I said in another reply, "trust" is as much a social construct as it is a technical one.
Could systemd choose to read a seed file after it mounts the root file system? Sure, but then it's on systemd to believe that the seed file is always secure, even on IOT devices where the seed file might be imaged onto millions of devices with an identical value out of the box. Using a seed file means you also have to trust how the OS is installed; security is a holistic property involving the entire system's design and implementation.
Ultimately, it's not going to be up to kernel or systemd to tell users and system administrators what they should or shouldn't trust. If you trust RDRAND, and you're on x86, you can enable the config option or provide the boot command line flag, and you're all set. But I'm not going to tell you one way or another whether or not you should trust RDRAND. And even if I did, you could just reject my advice, either way. As I said in another reply, "trust" is as much a social construct as it is a technical one.