ResearchBitcoin Dev Archive BIPs
BIP 383: Multisig Output Script Descriptors
Applications, Specification, Deployed. By Pieter Wuille, Ava Chow. Never named in a subject line or discussed by number on the list.
In Bitcoin Core
First shipped in Bitcoin Core 0.17.0 .
Output Script Descriptors, and most of Script Expressions are implemented as of v0.17.0 (PR 13697).
From Bitcoin Core's doc/bips.md.
The threads, in order
No thread on the list names this BIP. Search the archive for its title if you think one should.