This is a BIP draft for the rawtr() script expression, following the style of BIPs 381-387. Bitcoin Core has implemented it since 24.0 and it has never had a BIP; @sipa noted that on #2050 in March.
While writing this I did some digging and found rawtr in more places than I expected. BIP 390 allows musig() inside it, and two of the BIP 390 test vectors are rawtr() descriptors, so a published BIP already points at an expression no BIP defines. Outside Core it's a gap people route around: rust-miniscript prints rawtr_not_supported_yet where a descriptor should be, and bdk-ffi's tests cite rawtr as BIP-386.
Mail list discussion: https://groups.google.com/g/bitcoindev/c/CCZN_qQ5C1s