Looks like 6.2.8 is the latest from what I see:
https://www.splunk.com/page/previous_releases/universalforwarder#x86_64windows
Windows 7, 8 and 8.1 (64-bit)
Windows Server 2003, 2003 R2, 2008, 2008 R2, 2012 and 2012 R2 (64-bit) 6.2.8:
splunkforwarder-6.2.8-275559-x64-release.msi
Release Notes
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