The textbooks all say that, but is it really true in practice? I suspect it's a bit of holdover conventional wisdom from an earlier era of software. Usually, when I've created a database for a particular app, it was never used by anything else besides that app (and manual queries, for which versioning is mostly not relevant).

You might choose to have a set of views and functions that are versioned separately from your app, for the same reasons you might choose to set up a microservice. But as with microservices, it should be a deliberate decision that you make because you're getting something out of it that outweighs the complexity costs and version-skew headaches; it should not be the default way to do composition.

It’s absolutely true. Nobody should be building a microservice architecture for a product without a proven market fit, monolithic development is just faster at this stage, and when you get around to scaling it once you outgrow it (which should be much later than conventional microservice wisdom agrees) the database isn’t the bottleneck anyway and hence stays monolithic (don’t confuse with adding shards).

Making two views and deprecating one is actually the simplest possible compatibility strategy - just like adding a new version of a function to a header.