You are right that SSS requires temporarily rematerializing the raw key at the moment of signing. Even so it would be a huge improvement:

> Which makes the exact situation that occurred here possible.

Not so. The situation here was operator errror and someone mistakenly committing signing key in cleartext to repo. So this exact situation would not be possible. They could also have a process of signing on a dedicated instance (possibly with its own shard) which would remove key exposure completely from operator machines. This is all achivable with existing tooling and without changing implementations on verifier side.

The first link shows how an actual threshold signature scheme for PGP could be constructed.