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TLS 1.2 was enabled on SharePoint server but supporting .Net 4.6 framework was not upgraded from .Net 4.5 for TLS 1.2 to function correctly.
We are still investigating when TLS was enabled in the farm and was it part of any Windows upgrade/Patch/kb installation.
Resolution.
In order to display RSS feed in SharePoint page, we upgraded .Net 4.5 Framework to .Net 4.6 and RSS Feed started functioning correctly.
Future Mitigation.
We are still investigating when TLS was enabled in the farm and was it part of any Windows upgrade/Patch/kb installation.