The spec as written says that an override does not need to be statically used by an entry point, to be allowed to be specified in the pipeline descriptor:

6. For each key → value in descriptor.constants:
   • 1. key must equal the pipeline-overridable constant identifier string of some pipeline-overridable constant defined in the shader module descriptor.module

This was only briefly discussed but it was an intentional choice: #1766 (comment)

However from gpuweb/cts#1991 and #3600 it seems like WGSL has been assuming that it does need to be statically used. I'm not sure if the two specs technically conflict since WebGPU does not use the "interface of a shader" in the validation rule, but they definitely sound like they conflict.