We have scenarios, master project is in Indigo version but its client projects are in classic and want to update all client projects from classic to Indigo..

Hi Jing, First of all, please be so kind and try to keep all related comments under the one thread. It help us to keep the channel clean and organized. 😉 Not sure about this particular FeatureFlag/PlatformSetting and its behaviour within LCM, but you can simply enable it on domain level via API or gray pages. This way it would be hierarchically propagated to all tokens and workspaces, which belong to this domain.