The resource WelcomePluginsRootResource does not provide any JAX-RS annotated methods. This causes deployment to fail for some containers, e.g. TomEE providing CXF as JAX-RS implementation. CXF checks resources for JAX-RS spec compliance and throws an error if a class annotated with @Path does not provide any annotated methods.
I think the issue here is that WelcomePluginsRootResource is extending AbstractWelcomePluginResource, but it should extend AbstractWelcomePluginRootResource (note the "Root").
At least all other application do it like this, see CockpitPluginsRootResource for example.