LB extensibility was originally described in #5598. The infrastructure for this has been implemented, but there are a few missing pieces. Here's the relevant history:

• A new field for LB policy extensibility was added to xDS in cds: Add general-purpose LB policy configuration #7744.
• The extension mechanism was added to Envoy in Enable load balancing policy extensions #17400.
• The semantics of the xDS field were changed slightly in Prefer the load_balancing_policy cluster field over lb_policy #18419.
• xDS extension protos were added for ROUND_ROBIN and RING_HASH in xds: add config for wrr_locality and round_robin LB policy extensions #19517 and xds: add config for ring_hash LB policy extension #20423, but Envoy does not yet actually implement configuring those policies as extensions.

I think the remaining work here is:

• Add extension protos for all built-in LB policies.
• Implement actually configuring these policies via the extension protos.

Once this is done, we can consider whether we want to deprecate the existing enum for configuring built-in policies.