I remember installing XP on a FAT32 partition when it was first launched, but never tried it since then with subsequent versions of Windows. Is it still possible to do so with Vista onwards, or was FAT32 boot support dropped somewhere down the line? I'm looking for official documentation regarding this change.
Yes after Windows XP FAT32 partition system is not supported by the Windows installation.
Microsoft KB confirms it why user can't upgrade from XP to Vista if they are on FAT32. So from there they have leave the FAT32 boot system.