The Reactive Manifesto is certainly relevant for microservices. But a microservice does not have to be implemented with reactive programming in order to achieve the goals of the Reactive Manifesto.

Whether or not a microservice is implemented with reactive programming can be different for each microservice.

This can be a micro architecture decision and therefore affects only individual microservices, but not the system as a whole.

It is important to understand the difference, because otherwise the choice of technologies might be limited to reactive programming frameworks even though that is not necessary.

It is perfectly fine to stay with established technologies. In fact, using a technology stack that you are used to might be easier and bring faster results.

At the same time, it is possible to try new technologies like reactive programming in one microservice and then use it in other microservices if it has proven to be useful.