You always have this problem thats why you have a release process for apis.

And monorepo or not, bad software developers will always run into this issue. Most software will not have 'many teams'. Most software is written by a lot of small companies doing niche things. Big software companies with more than one team, normally have release managers.

My tipp: use architecture unit tests for external facing APIs. If you are a smaller company: 24/7 doesn't has to be the thing, just communicate this to your customers but overall if you run SaaS Software and still don't know how to do zero-downtime-deployment in 2025/2026, just do whatever you are still doing because man come on...