Is it true that I can’t use plugins in vSphere Client any more?
vSphere Client supports plugins with the plugin SDK.. If you are missing plugins that you used in the past, check if the plugin is still needed and/or check with the provider of that plugin if there is a HTML5 compatible plugin available.
On the VMware Code pages you will find the vSphere Client SDK. https://code.vmware.com/web/sdk/6.7/client