Configured the Console > Settings > Agents/Plug-ins > All Agents/Plug-ins > Software > Patch Management > Windows > Default Software Update Plug-in Policy (DSUP) with the following settings for reboot notifications and deferral:
Confirmed when the Client executes the Software Update Cycle on schedule per the DSUP the Symantec Management Agent (SMA) delivers the Deferral Popup without issues; however, if the Postpone button is not selected within under a minute, the Postpone button is removed from the popup as if it had reached the max deferral time and the only option is to Restart Now.
Patch Management 8.0 HF4
Confirmed configurations were causing conflicts in processes which resulted in Postpone button disappearing.
This issue was resolved in the ITMS 8.0 HF6 and confirmed resolved in future ITMS 8.1 release.
Please implement this HotFix (HF) 6 to resolve this issue as detailed in DOC9693.