When we were using classic projects, we were able to generate ruleapp using RuleApp project in rule designer. But how can we generate the ruleapp jar files from the Rule Designer for the decision services?

You can use the deployment configuration to export the rule application from the Rule Designer. You can find more details about the the deployment configuration from this link:

https://www.ibm.com/support/knowledgecenter/SSQP76_8.9.2/com.ibm.odm.dserver.rules.deploying/shared_cmg_topics/con_cmg_deployconf_overview.html

By this link, you can define a deployment configuration that includes the following information:

A configuration type: production or nonproduction. This can be used to limit deployment to certain servers.

The RuleApp name that is used by the client application to request a decision.

The decision operations with the rules to be deployed.

The target servers.

The ruleset version base number and versioning policy.

But for your purpose of exporting a rule application by a deployment configuration:

1. give this configuration a specific name for this purpose, such as "ruleapp export configuration".

2. don't define the target server mentioned above, leave this part empty.

3. then on the "Overview" tab, click on the "Proceed to Ruleapp deployment...", a ruleapp jar file will be generated.