alt Hacker Newsif service A calls service B, and service B adds a new endpoint, or a new optional argument, service A needs an update to take advantage of it
if a library is used by multiple services, and gets an important bug fix, each service using the library needs to update to get the fix
these are sequences of changes, not literally at the same time or requiring deployment coordination, but when this happens a lot people start asking about monorepos
But a monorepo wouldn't make any of that easier.
You have to make the change to service A and B, and then test both and deploy both. You haven't saved any time or effort in a monorepo.
A library needs updating, you still have to update it and then test and deploy every service that relies on it independently. Again you haven't saved any time or effort. In fact you've made it worse, because if those services are maintained by different people, you just forced them to test and deploy on your timeline and priority, not theirs. You actually made the coordination problem worse.