Build backwards compat for `multiValueHeaders` and `multiValueQueryStringParameters`?

[ryanblock]

API Gateway HTTP APIs don't support REST API params of multiValueHeaders or multiValueQueryStringParameters.

Format 2.0 doesn't have multiValueHeaders or multiValueQueryStringParameters fields. Duplicate headers are combined with commas and included in the headers field. Duplicate query strings are combined with commas and included in the queryStringParameters field.
See: https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-develop-integrations-lambda.html#http-api-develop-integrations-lambda.proxy-format

I'm wondering if it's feasible to enable backward compat for those params in HTTP APIs to ease upgrades – given the fact that duplicate headers are combined with commas, however, that tells me we are unlikely to be able to do so in a non-lossy, deterministic fashion.