ResearchBitcoin Dev Archive BIPs
BIP 373: MuSig2 PSBT Fields
Applications, Specification, Complete. By Ava Chow. Never named in a subject line or discussed by number on the list.
In Bitcoin Core
First shipped in Bitcoin Core 30.0 .
MuSig2 fields for PSBT as of v30.0 (PR 31247)
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.