That PR has been closed and within the comments are the solution to getting proxy protocol enabled already in Istio.
First, proxy protocol needs to be enabled on the ELB: https://github.com/istio/istio/pull/12748#issuecomment-576491574
Then, an EnvoyFilter needs to be added to enable it within Istio: https://github.com/istio/istio/pull/12748#issuecomment-576488018
I haven’t tried it myself yet, but figured I’d update this thread since it was the result that came up when I googled for the same thing.