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In the newest release of the "SharePoint Online Management Shell" (v16.0.4630.1200) the included Microsoft.SharePoint.Client.dll adds the property RequestAccessEmail to the Web class.

Download: https://www.microsoft.com/en-us/download/details.aspx?id=35588

You may use this DLL until the NuGet-Package is released.

EDIT: Since version 16.1.4727.1200 of the Microsoft.SharePointOnline.CSOM NuGet-Package the RequestAccessEmail property is supported.