Terraform: Conditional statement blocks in aws_iam_policy_document?

Yes. You can use a dynamic block with a boolean to optionally include the block.

data "aws_iam_policy_document" "policy" {
  statement {
    sid = "PolicyAlways"

    ...
  }

  dynamic "statement" {
    # The contents of the list below are arbitrary, but must be of length one. 
    # It is only used to determine whether or not to include this statement.
    for_each = var.enable_optional_policy ? [1] : []

    content {
      sid = "PolicySometimes"
      ...
    }
  }
}