Does AWS ELB support load balancing to on-premises targets?

Yes, both ALB and NLB supports load balancing to IP addresses as targets for AWS and on-premises resources.

Reference:

https://aws.amazon.com/about-aws/whats-new/2017/08/elastic-load-balancing-application-load-balancer-now-supports-load-balancing-to-ip-addresses-as-targets-for-aws-and-on-premises-resources/

https://aws.amazon.com/about-aws/whats-new/2017/09/elastic-load-balancing-network-load-balancer-now-supports-load-balancing-to-ip-addresses-as-targets-for-aws-and-on-premises-resources/