The thread_type field was introduced way back in May 2000 on Linux as it was needed for the newly started thread to determine whether it had to install (and later remove) an alternate signal stack. When we later stopped using alt-signal-stack (I'm going to guess this was when we switched from LinuxThreads to NPTL) the field was no longer used, but meanwhile we used the ThreadType enum to control initial stack sizes.

Windows never had a thread_type field and BSD/AIX likely just blindly copied it from the Linux code.

Attaching a thread never involved using alt-signal-stack so for attached threads it was left uninitialized.