The Core Lightning developer Dusty Daemon created splicing. He announced the proposal back in 2021. It alters BOLTs 2, 7, and 9. You can see the pull request here. Dusty has already made a splice on main net, as you can see here. Currently, multiple Lightning Protocol implementations are working on Splicing, such as CLN and Eclair. Besides the UX improvements that Splicing offers, it has great potential to increase Bitcoin’s privacy, as the splice transaction can be used as a coinjoin. You can read more about it here.