The actual algorithm (which is pretty sensible in the absence of delayed ack) is fundamentally a feature of the TCP stack, which in most cases lives in the kernel. To implement the direct equivalent in userspace against the sockets API would require an API to find out about unacked data and would be clumsy at best.

With that said, I'm pretty sure it is a feature of the TCP stack only because the TCP stack is the layer they were trying to solve this problem at, and it isn't clear at all that "unacked data" is particularly better than a timer -- and of course if you actually do want to implement application layer Nagle directly, delayed acks mean that application level acking is a lot less likely to require an extra packet.

If your application need that level of control, you probably want to use UDP and have something like QUIC over it.

BTW, Hardware based TCP offloads engine exists... Don't think they are widely used nowadays though