ln order to deploy our services over docker swarm we use 2 different yml files : the base one and the customized one
Here it is an example of our base.yml file:

version: '3.6'
services:
    hello-server-api:
      image: hello-server
      environment:
        - ID=1
      deploy:
        mode: replicated
        replicas: 1
        update_config:
          delay: 50s
        restart_policy:
          condition: on-failure
          max_attempts: 3

Here it is an example of our customized.yml file:

version: '3.6'
services:
    hello-server-api:
      deploy:
        mode: replicated
        replicas: 0
        update_config:
          delay: 50s
        restart_policy:
          condition: on-failure
          max_attempts: 3

we use the customized yml file to manage variables from production and the number of replicas of each container.

docker command to deploy:

docker stack deploy -c base.yml -c customized.yml server

This approach on docker-ce 19.03.11 was working correctly, in fact, the docker service is always deployed 0/0.

Upgrading the docker-ce version to 20.10.0 (we also tried 20.10.8) the behavior is changed and the replicas are always 1/1, it seems like it is not possible to set 0 in the customized.yml file.

However, if we put replicas: 2 in the base.yml and replicas: 1 in the customized.yml it is working correctly (it is working properly in every increasing and decreasing scenario, except when 0 value is used in the customized one ).