AppSync VTL "hasCustomPartitionKey" Not Working

I've added hasCustomPartitionKey to a putItem resolver with a data source configured with sync. However, I'm getting the following TemplateMapping error message: "Unsupported element '$[hasCustomPartitionKey]'."

This is the operation:

{
"version": "2018-05-29",
"operation": "PutItem",
"_version": 1,
"key": $util.toJson($modelObjectKey),
"hasCustomPartitionKey": true,
"attributeValues": $util.dynamodb.toMapValuesJson($ctx.args.input),
"condition": $util.toJson($condition)
}

Am I doing something wrong here ?

Start with the AWS AppSync VTL PutItem resolver documentation and the shown operation, focusing on hasCustomPartitionKey with a sync data source. Verify the accepted operation fields and reproduce the TemplateMapping error; done means identifying whether the field is supported and recording the required resolver change or documentation.

Written by the indexing model from the issue text.