No breaking changes have been introduced to Hono's external APIs, existing clients can therefore still be used with Hono 2.0.0.
However, there are a few breaking changes to the way that some aspects of the components are configured. This is mainly due to the fact that all components are now based on the Quarkus framework which already covers some of the aspects like health checks and Vert.x configuration which had been configured in a more generic way before. The corresponding changes are reflected by the 2.0.0 release notes.
In addition, many internal classes have been moved to different modules and many modules have been renamed and/or removed as part of the migration from Spring Boot to Quarkus. This should have no impact on the external behavior of a Hono installation but might require refactoring of code that depends on Hono's internal classes.