As a developer you have to make choices of what versions of NopCommerce you want to support for plugins.
It would be nice to have some stats on what versions of NopCommerce are being used on current live sites so that you can make a choice of what versions are worth supporting when it comes to plugins.
Is there such a list?, if so where can I find it?
Officially the is no such DOC.But From Nopcommerce4.0 version you can define multiple Nopcommerce versions for a plugin like "SupportedVersions": [ "4.00","4.10","4.20" ]. But from the public, it is tough to say which exact Nopcommerce version is using for a specific site. In the admin site very right bottom there is a version number.
For what it's worth, I'd start with the latest version. Then, if someone finds your plugin but needs it for a prior version, they would ask you for it and then you can decide.