[Deukhoofd]

> That means you need to know which version the tool you use supports

Honestly the same issue with versioning has been my primary issue with XML Schemas in the past. XSD 1.1 for example came out over a decade ago, but is still very badly supported in most tooling I tried out.

> When we are talking about a client program calling a server API with JSON/XML, why do we care about the format of data exchanged?

We shouldn't care much, beyond debuggability (can a developer easily see what's going on), (de)serialization speed, and bandwith use. JSON and protobuf tend to be a decent chunk smaller than XML, JSON is a bit easier to read, and Protobuf is faster to (de)serialize. This means they should generally be preferred.

In the case of a client program calling a server API I'd personally have the server do the required validation on a deserialized object, instead of doing so through a schema. This is generally easier to work on for all developers in my team, and gets around all the issues with tooling. The only real reason I use schemas is when I'm writing a file by hand, and want autocompletion and basic validations. In that case versioning and tooling issues are completely in my control.