SharePoint CSOMs for Customizations
- CSOM version 15 is for SharePoint 2013
- CSOM version 16 is for SharePoint 2016
- CSOM version 16.1 is for SharePoint Online
Therefore when you are making a development against a specific platform, it is best and recommended to pick the correct version.
I was mistaken…
Also it is mentioned in the blog why it works,
It works fine unless you will use properties or methods which are not exposed in the on-premises. If you can guarantee that your code does not touch properties which have not been enabled in on-premises version, you can theoretically use SharePoint Online CSOM with on-premises as well. We do however recommend on using right CSOM version for specific server side version to avoid issues.
Microsoft Download Center has all the client component SDKs related to each version of SharePoint.