We are currently targeting the Windows API of Windows NT 4.0 SP3. That
version’s lifecycle ended in January 1999 (22 years ago!). 
Some win32 functions we are (optionally) using are not available with that target API. To work around that and to possibly allow using these functions on Windows versions where they are available, we currently use
GetProcAddress on runtime (instead of linking the respective libraries directly). That leads to less efficient and more fragile code.
Additionally, gnulib uses different (maybe more efficient?) functions depending on the target Windows version, IIUC.
It was previously reported that some of the dependencies we build with MXE Octave (Qt IIRC) no longer run on Windows XP and older.
Would it be ok to increase the Windows API version we target in Octave to Windows 7?
The support by MS for that version of Windows ended in January 2020 . But it still seems to be used by a considerable share (18% as of December 2020 ).
The newest Windows version that we wouldn’t support any longer after that change would be Windows Vista for which the support ended in April 2017. 
That version seems to have a very low share in the meantime (below 0.5% according to the same statistics).
See also bug #57270 where this came up before.
: Search product lifecycle (microsoft.com)
: Windows 7 support ended on January 14, 2020 (microsoft.com)
: Desktop Windows Version Market Share Worldwide | StatCounter Global Stats
: Windows Vista support has ended (microsoft.com)