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.