I enabled pub/sub in my Google Cloud functions account, but subsequently realized that CloudTasks is a better solution for my use case. But when I went to disable it, I got the message that cloudbuild.googleapis.com, cloudfunctions.googleapis.com and containerregistry.googleapis.com would all be disabled because they depend on pubsub.
I use cloud functions (via Firebase) quite heavily, and certainly don't want to disable that though.
Did I just walk through a one way door when I enabled pubsub?
As shown in the documentation, in order to work with Cloud Functions, you would need to enable multiple APIs. Among these is included the Pub/Sub API. Merely having this API enabled should not create any issue in your project, nor generate charges unless billable resources are created.