The earliest firmware version available for these switches were released 21-SEP-2010 for the SG300-28 and 25-JAN-2011 for the SG200-26 so these should be close to their release date one would think. Why do you want to know when they were released?
Sx550X, Sx350X, Sx250: PSE will Supply Power to Catalyst PSE Ports
May 31, 2016
June 5, 2017
Configure Remote Network Monitoring (RMON) Events Control Settings on a Switch through the Command Line Interface (CLI)
Remote Network Monitoring (RMON) was developed by the Internet Engineering Task Force (IETF) to support...