At the page linked below while the value of 'ServicePointManager.Expect100Continue' is true by default it does not appear to be wired up and the behavior detailed under Remarks does not occur. Specifically for .NET Core versions.

This doc should be updated to reflect this

Additional details can be found at...
dotnet/runtime#17336

Document Details

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https://docs.microsoft.com/en-us/dotnet/api/system.net.servicepointmanager.expect100continue?view=net-5.0&source=docs