Talked to NPM about this when it went viral, and once again all they could say was enabling package signing, even optionally, would discourage inexperienced people from contributing packages as they would be pressured to learn basic security and key management.
Sorry, if you are unwilling to take 10 minutes to learn how to sign your code, you have no business maintaining software releases millions of people depend on. Full stop.
But NPM will not even make it -optional- and rejects all PRs to implement this going back a decade. So no one feels pressured into basic security. Cool.