Maxim's get_seq_commit_lane_proof RPC has now been merged into Rusty Kaspa. Sutton announced the merge on June 14, linking PR #961 and tagging Maxim and we_are_legion. (source) (PR #961)
The new endpoint returns the witness a client needs to verify one KIP-21 active lane against the seq_commit in a block header without running a full node. (PR #961) The response includes the SMT proof bytes, optional lane tip and lane blue score, plus the remaining metadata needed to chain the lane root into the sequencing commitment. (PR #961)
This is a real update on an older KIP-21 topic, not another high-level recap. The earlier story was that clients would need a proof surface for lane verification; the new fact is that the gRPC and wRPC endpoint landed with implementation, serialization tests, service wiring, and an integration test covering both absent and existing lanes. (PR #961)
All sources link to public messages in the Kaspa Core R&D (public) Telegram channel.