PROXY protocol is only available on HTTP load balancers (REGIONAL type). Network load balancers (REGIONAL_NETWORK type) do not support it because they preserve the client source IP address natively.
In DOKS 1.33.1-do.0 and later, the default load balancer type is REGIONAL_NETWORK. If you need PROXY protocol, you must explicitly set the load balancer type to REGIONAL by adding the annotation service.beta.kubernetes.io/do-loadbalancer-type: "REGIONAL" to your service configuration.