SIGINFO on GNU Linux (Arch Linux) missing

There was talk (back in the linux 0.x-1.x days) of adding this (because it was useful on BSD systems) but if I recall correctly there were reasons it was harder to do right on Linux than BSD at the time.

Note that what you're asking about is only a small part of the feature (namely, you're talking about an stty info entry for control-T causing the kernel to deliver SIGINFO to the tty's process group) - that part is "easy" - but having the kernel report information about the process status when it doesn't handle the signal (because at the time very few things had any support for that, the feature was mainly about "is this process spinning or hung" and "what process is it anyway") is harder - ISTR there even being security/trust issues about displaying that information accurately, and whether it should be associated with the Secure Attention Key path. That said, there might be some value in the "easy" version that only sends the signal...

(From personal memory; a quick web search doesn't turn up anything obvious but I think one would have to dig into really old archives to find the discussion.)