Threshold signatures are mentioned in the BIP which started the thread, and by combining Schnorr signatures with Pedersen Secret Sharing, it is possible to obtain an interactive threshold signature scheme that ensures that signatures can only be produced by arbitrary but predetermined sets of signers. For example, k-of-n threshold signatures can be realized this way. Poelstra also mentioned that the combination of MuSig and VSS is implemented in his code. Erik Aronesty raised questions about building up threshold signatures via concatenation or indicating that of M of N are valid, but Poelstra did not address those specific points.