> Storing multiple projects in a single repository does not magically make them a single unit of deployment, even when you deploy them all with a single pipeline. When you have multiple projects, you always have multiple units of deployment.

It makes it easier to turn them into the same unit of deployment. There's nothing you cannot do some other way of course.

You're right about atomic deployments being difficult and sometimes one can control that risk by the order in which you change things. In a monorepo it's slightly easier to record some kind of script, makefile, dependency system that says "deploy this before that".

With browsers -for sure your user level API has to be stable and cannot change in a sudden incompatible way. When people have seen fit to have layers of APIs underneath it though, one can still have a lot of change thats theoretically hidden from users but still changes lots of APIs.