The Office add-in I am working on will be supported only for the Office Online platform. There are configuration to set Host in which the add-in will work, that can be added in manifest.
Is there a similar setting/configuration available in manifest to explicitly mention that the add-in will support only Office Online platform and not Office for Windows, etc.
If you are planning to publish your Office Add-in to the Office Store (which I assume you are, since you added the office-store tag to your question), it must comply with all Office Store Validation Policies.
As stated in section 4.12.1 of that documentation:
Your add-in must work across all platforms that support the methods that you define in the Requirements element in your add-in manifest.
In other words, if your add-in uses requirement sets and methods that are supported cross-platform, then your add-in must work cross-platform.