@sudeshgaur,
My support case on this issue was resolved -- we found that the file was being put into /Library/LaunchAgents, and instead it needs to be in /Library/LaunchDaemons. Our boot-start worked at that point, and I believe Splunk updated the documentation regarding this.
Also be sure you have the following:
-Permissions on the plist (0644)
-Verify owner on the Application (chown -R yoursplunkuser /Applications/SplunkForwarder.app)
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