Most of the examples of using Spring Modulith use DDD in some capacity and organizing things by bounded contexts.
My use case is an application which offers a REST API which interacts several backends (our ERP SDK, an external REST API, an external GraphQL API).
My initial thought was to organize this into modules that interact with each particular backend. One module for the SDK, one module for the external REST API, etc. However, is this an invalid approach?
As long as you're fine with organizing modules by domain, there's nothing that strictly requires anything DDD-related. Spring Modulith primarily focuses on the notion of application modules and how to decouple them in various degrees.