Giving an alternative Async method to a service
If your implementation does not create interrupts of some sort there is no point in making it async. If the consumer does not want to wait for your code to execute its better to let them control that and execute your code seperate from the main thread.
Edit: if the interface and implementation are shipped seperate or if its possible someone will replace default none async implementation with a async implementation. Than it could be a good practice to make the interface Async and in your default implementation return Task.FromResult or Task.CompletedTask.