AWS provider 2.0.0 disables the function

[raymondbutcher]

https://github.com/terraform-providers/terraform-provider-aws/blob/master/CHANGELOG.md#200-february-27-2019

resource/aws_lambda_function: Setting reserved_concurrent_executions to 0 will now disable Lambda Function invocations, causing downtime for the Lambda Function. Previously reserved_concurrent_executions accepted 0 and below for unreserved concurrency, which means it was not previously possible to disable invocations. The argument now differentiates between a new value for unreserved concurrency (-1) and disabling Lambda invocations (0). If previously configuring this value to 0 for unreserved concurrency, update the configured value to -1 or the resource will disable Lambda Function invocations on update. If previously unconfigured, the argument does not require any changes. See the Lambda User Guide for more information about concurrency.

[raymondbutcher]

Resolved by #39