According to Wikipedia, Qt for Windows isn't FOSS until version 4.
The last version of Qt to support targeting XP and Vista is 5.6.
| 95 | 98 | Me | NT4 | 2000 | XP | |
|---|---|---|---|---|---|---|
| 2.3 | yes | yes | yes | yes | yes | |
| 3.3 | yes | yes | yes | yes | yes | yes |
| 4.3 | yes | yes | yes | yes | yes |
4.4 indicates support for compilers only, not OSes. It's the last version to support Visual C++ 6.0 and 2002 (7.0).
4.7 has claimed support for Windows XP to 7, using MSVC 2005 (8.0) and 2008 (9.0).
4.8 has claimed support for Windows XP SP3 and 7, but not Vista.
5.15 still supports Windows 7 and 8.1, but 6.0 only supports Windows 10.
Since 6.2, only version 1809 or later of Windows 10 is supported. Windows 11 also started to be supported.
6.3 temporarily raised the requirement to 21H2 or later. The documentation of 6.4 states a nonsensical "21H2 (1809 or later)". 6.5 reverted it to 1809 or later.