Yeah, POSIX IPC is close, but not quite. While SysV shared memory is old, it still provides features that are not available anywhere else. The one I can think of off the top of my head is nattach, being able to examine a segment and see if it is still in use and by who.
This comes up every now and then when people want PostgreSQL to use POSIX shared memory or mmap(). Turns out that there is no portable replacement for all the features of SysV shared memory. Which means you could do it, but you lose a number of safety-interlocks you have now. And safety of data is critical to databases.