The Windows NT series DDK (as well as the MinGW/ReactOS implementation) includes a struct NT_TIB in winnt.h that documents the subsystem independent part.
Through the pointer to the PEB one can obtain access to the import tables (IAT), process startup arguments, image name, etc.
The TIB contains the thread-specific exception handling chain and pointer to the TLS (thread local storage.)
The deallocation stack is important because Windows API allows to change the amount of guarded pages: the function SetThreadStackGuarantee allows both read the current space and to grow it.
Using Microsoft Windows SDK or similar, a programmer could use an inline function defined in winnt.h named NtCurrentTeb which returns the address of the current Thread Information Block as NT_TIB *.