LEDBAT typically runs over UDP and doesn't need kernel support. Windows has something similar with "background intelligent transfer service" BITS, but LEDBAT works just fine outside kernel.
But you can apply it to TCP as well (only at a kernel/raw socket level), which Apple and Microsoft have already done, which makes it simple for software to use; Apple uses it to download o/s updates in the background.
Implementing it in the kernel also makes programs like “trickle” much easier.
Implementing it in the kernel also makes programs like “trickle” much easier.