Seldon Core v1 has features for traffic-splitting, which they rely on a service mesh as a gateway. It supports Istio or Ambassador. Seldon Core v2 is not reliant on an external dependency for traffic-splitting.
Detector components in Core v1 use Knative, which is also dependent on a service mesh for traffic routing. You can find the supported technologies in the Knative documentation.
Seldon Deploy itself does not have a hard dependency on Istio. It has dependencies on components that use Istio.
Seldon Deploy can be used to make predict calls to models. The gateway affects the form of the URLs to the models. However, this is configurable in the Seldon Deploy values file using templating to set the form of the URL from relevant parameters. It defaults to Istio.