These operating systems are no longer supported *by Microsoft* and haven't been for a while. There are no more security updates, no more patches, no more support, no more anything. Has the department at your company responsible for this delved into any sort of a risk assessment on this situation?
Any UF you can install on them will be, by its nature, also unsupported too.
That being said, the latest UF I could find supporting Windows 7, was 6.4.11, which really is absolutely ancient.
(If you flip the version drop-down in the upper right to the next update, 6.5.0, it no longer lists Win7).
Have you tried just installing, say, the latest in the 7.3.x series of UFs? I mean, who knows? Nothing you do will ever be supported (neither MS nor Splunk supports Win7 now, regardless of what version of the UF or Splunk you have), so why not just try it anyway?
I doubt 8.x will work (though you could try), but 7.x at least has a chance.
Server 2008 R2- you don't mention service pack.
For no service pack, I think this is the latest, another pretty ancient version: