Online PSDK documentation states 5.1 (WinXP):
To compile an application that uses this function, define _WIN32_WINNT as 0x0501 or later. For more information, see Using the Windows Headers.
Requirements
Minimum supported client: Windows XP [desktop apps only]
Minimum supported server: Windows Server 2003 [desktop apps only]
Older local copy of PSDK doc vaguely disagrees with above, and attempts to state either 5.0 (Win2k) or 5.1:
To compile an application that uses this function, define _WIN32_WINNT as 0x0500 or later. For more information, see Using the SDK Headers.
Requirements
Client: Requires Windows Vista or Windows XP.
Server: Requires Windows Server "Longhorn" or Windows Server 2003.
Finally, PSDK headers (rather old too) are advocating for Win2k:
#if(_WIN32_WINNT >= 0x0500)
WINBASEAPI
BOOL
WINAPI
AttachConsole(
__in DWORD dwProcessId);
#define ATTACH_PARENT_PROCESS ((DWORD)-1)
#endif /* _WIN32_WINNT >= 0x0500 */
I dont have anything older than WinXP SP3 handy to examine kernel32.dll export directory, so please enlighten me what is minimum supported version for real, 5.0 or 5.1?