Google famously still primarily maintains a monorepo, but they absolutely don't deploy a monolith. They published a paper last year that describes an approach that is quite close to what they have internally - https://dl.acm.org/doi/pdf/10.1145/3593856.3595909

Effectively, by setting strong boundaries between modules, they can develop on their application as if it was a monolith, but at runtime, separate those modules physically deploy them as micro-services. This solves a lot of the versioning issues (since the app is rolled out atomically) and correctness issues (easier to reason through a monorepo).