If the <!--ih:includehtml file="mycode.txt"-->(codecodecode)<!--/ih:includehtml--> is in a document that is being updated but the "mycode.txt" file is not in the folder, what will happen to (codecodecode) in the document? In other words, will the (codecodecode) be deleted?

Actually, includeHTML will raise an error because it cannot find mycode.txt, and the file will not be processed.