demoData option was lastly present in syndesis 1.2, that means even before Fuse 7.0:

https://github.com/syndesisio/syndesis/blob/1.2/app/rest/dao/src/main/java/io/syndesis/dao/manager/DataManager.java#L75-L76

since then the property isn't anywhere in the code, so with a high probability, it doesn't do anything

EDIT: or as suggested by zregvart-redhat: "We might want to make this bit conditional on that setting then: https://github.com/syndesisio/syndesis/blob/0e96516d1bef4d510ef40dba374d3a567d923b2a/app/server/dao/src/main/resources/io/syndesis/server/dao/deployment.json#L2-L20"