I am using the REST API PATCH call to update a Contact like so:

URL: salesforce/data/<version>/sobjects/Contact/<externalidentifier>/<value>
JSON: map of fields to values

In addition to settings some values, I'd like to set others to null. I tried to pass fieldsToNull = [string, string, string] in the JSON.

However this results in a response saying:
"No such column 'fieldsToNull' on sobject of type Contact."

I also tried appending ?fieldsToNull=<field> to the request URL, which than returned successfully, but had no affect in respect to settings the field to null.

All documentation I can find providing examples with fieldsToNull are for soap/wsdl and not rest.