I need to execute the extend line tool programatically. I'm Using ArcObjects .NET SDK 10.8. In ArcMap this tool is available in the ArcToolbox under the Editing Menu. Is this tool available in ArcObjects SDK?
You just need to execute the gp tool with the correct parameters.
How to run a geoprocessing tool: https://desktop.arcgis.com/en/arcobjects/latest/net/webframe.htm#RunningGPTool92.htm
The tool reference for extend line: https://desktop.arcgis.com/en/arcmap/latest/tools/editing-toolbox/extend-line.htm