When developing a shopware plugin i want to define which SW versions my plugin is tested and compatible with.
What is best practice in terms of marking my plugin for version compatibility?
In the composer.json of your plugin you can use the require property to set the version of the Shopware core package you require.
"require": {
"shopware/core": "~v6.5.0"
}
See the documentation on how to specify the required version.
In addition you can also add the shopware/administration and/or shopware/storefront packages to be required, if they are needed for full functionality of your plugin.