On a serverless platform, a runtime upgrade must never break a live application, because developers have no control over when upgrades happen.

In the old world, if the Node.js maintainers decide to make a breaking change to an obscure API between releases, it's OK. Downstream developers are expected to test their code before upgrading, and address any breakages. But in the serverless world, it's not OK: developers have no control over when upgrades happen, therefore upgrades must never break anything.