Not sure how north korean hackers benefit from not knowing who is accessing their servers?

We have Cloudflare Tunnels to thank for this. I wish Cloudflare did more to stop malicious use of Cloudflare Tunnels.

I thought ohttp is opt-in. You have to participate their beta to enable the relay/gateway.

Same applies to Apple's Private Cloud Compute. A service provider has to join the program to avoid reading visitor's source IP.

It will handicap service provider's capability if I am not mistaken.

I think you misunderstand. This is for inbound traffic, not outbound.