This somewhat reminds me of the old MakeProcInstance mechanism in Win16, which was quickly rendered obsolete by someone who made an important realisation: https://www.geary.com/fixds.html

Another seemingly underutilised feature closely related to {Get,Set}WindowLong is cbClsExtra/cbWndExtra which lets you allocate additional data associated with a window, and store whatever you want there. The indices to the GWL/SWL function are quite revealing of how this mechanism works:

https://learn.microsoft.com/en-us/windows/win32/api/winuser/...

All my window classes uses cbWndExtra, and I leave GWLP_USERDATA for the user who is creating windows.