有没有办法扩展SharePoint客户端对象模型并添加ListItem.SystemUpdate方法？
No. CSOM 不可扩展。
更新方法在CSOM中可用！！
CSOM 包是 16.1.5626.1200
https://developer.microsoft.com/en-us/office/blogs/new-sharepoint-csom-version-released-for-Office-365-august-2016-updated/
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