> His points were that the Open-Closed principle isn’t very important anymore because most of the code we write isn’t contained in large monoliths and making changes to small microservices is safe and easy.

IMHO, Open-Closed applies just as well to entire microservice architectures as it does to individual classes. For example, I've seen services for general-purpose tasks such as job scheduling or generating PDF reports which required devs to change the code to add a new job type or report type. These services were replaced by services where the new application-specific types could be defined through an API, which meant that those services were "done" and reduced the amount of work required to use them.