What previous "bad IPC" mechanisms were integrated into mainline Linux? Only one I can think of is sysv shared memory.
All of them :)
Pipes are too specialized, localSockets are kind of a pain, etc.
Most recently, kdbus. It wasn't even ABI-compatible.
kdbus was never merged to mainline.
Sorry, I took 'integrated' to mean something other than merged. No, kdbus was never part of the kernel repo.
The reason Linux is opposed to integrating IPC into the kernel is because it's been done more than once, poorly, resulting in a lousy API that needs to be supported forever by kernel devs.
So come up with a good solution.