No where does it say time_frame is a post body object rather than key pair, this actually will NOT run as url parameter. Please take note this is poor consistency as filter DOES accept JSON via key-value param, so there is an expectation time_frame (also technically a filter, should too). 

Response received when running as parameter 
{
    "message": "{\n  \"time_frame\": {\n  \"from\": \"2020-01-01\",\n  \"to\": \"2024-12-31\"\n  }\n} is not a valid time_frame"
}

Get Custom fields documentation disregarded the required namespace parameter 

Updating a namespaces custom field on a record does was ambiguous with the field <id>.