The Rust-Lightning v0.3-rc1 release introduces significant API changes, including support for splicing with Replace-By-Fee (RBF), async signing, and BOLT 12 PayerProofs. The update also enhances performance with parallel reading of channel monitors and updates. Notably, the release includes bug fixes for channel gossip propagation and HTLC handling. Backwards compatibility is affected, with changes to payment metadata and BOLT 12 refunds.