It looks like the limitation related to security groups with ingress/egress blocks not being able to be distinguished from managed rules also affects route tables and managed routes.
We have quite a few AWS route tables where some of the rules are added via Terraform aws_route resources, but some have been manually added.
driftctl does not report any of the manually added routes.
When I look in the JSON state file, the manually added routes are present in the managed aws_route_table, and so I guess that's why driftctl can't distinguish whether those rules got there via route blocks in the aws_route_table, or via aws_route resources.