That very much depends on your definition of service.

OASIS defines a service as "a mechanism to enable access to one or more capabilities, where the access is provided using a prescribed interface and is exercised consistent with constraints and policies as specified by the service description". [0]

I see nothing that requires synchronous communication only. Virtually every large micro-service architecture I have ever seen communicates through queues asynchronously.

[0] https://en.wikipedia.org/wiki/Service_(systems_architecture)