Положение дел у Windows: сколько разношёрстных уровней UI в Windows 10?

недостоверная инфа

Windows NT includes a number of other environmental subsystems, each a Win32 application in its own right, to run other types of applications. These subsystems include:

Win16 subsystem
The Win16 subsystem, or Windows On Windows (WOW) as it's commonly known, is a Win32 application that provides a virtual DOS/Windows 3.1 machine. MS-DOS and Windows 3.1x 16-bit applications can run in this subsystem, with access to the complete Win16 API, and standard 16-bit Windows memory models. Most 16-bit DOS and Windows applications run perfectly within the WOW subsystem; the only exceptions are those applications that attempt to directly access system hardware and resources. Windows NT does not allow such applications to have direct access to those resources, in order to maintain system stability and performance (see Figure 21.9).

https://cmd.inp.nsk.su/old/cmd2/manuals/networking/perfomance/ch21/ch21.htm

Ну, и накину

NTVDM, or the NT Virtual DOS Machine, is a system component introduced in 1993 for all IA-32 editions of the Windows NT family (not included with 64-bit versions of the OS). This component allows the execution of 16-bit Windows applications on 32-bit Windows operating systems, as well as the execution of both 16-bit and 32-bit DOS applications. The Windows NT 32-bit user-mode executable that is the basis for a single DOS (or Windows 3.x) environment is called ntvdm.exe.

https://docs.microsoft.com/en-us/windows/compatibility/ntvdm-and-16-bit-app-support

Раз это 93-й год, то это точно не может быть NT4 — она вышла позже, значит, NT3.x, т.е. можно считать, что поддержка DOS (неполная, конечно) была в NT испокон веков

переписана с нуля поверх NT API

/****************************** Module Header ******************************\
* Module Name: MfPlay16.c
*
* This file contains the routines for playing the GDI metafile.  Most of these
* routines are adopted from windows gdi code. Most of the code is from
* win3.0 except for the GetEvent code which is taken from win2.1
*
* Created: 11-Oct-1989
*
* Copyright (c) 1985, 1986, 1987, 1988, 1989  Microsoft Corporation
*
*
* Public Functions:
*   PlayMetaFile
*   PlayMetaFileRecord
*   GetMetaFile
*   DeleteMetaFile
* Private Functions:
*   GetEvent
*   IsDIBBlackAndWhite
*
* History:
*  02-Jul-1991 -by-  John Colleran [johnc]
* Combined From Win 3.1 and WLO 1.0 sources
\***************************************************************************/