Read the following link, it will guide you to know about the VMware replacement process: HTML5 instead of FLEX
https://vinfrastructure.it/2018/01/html5-based-vsphere-client-replace-vsphere-web-client/
Anyway you may see the HTML5 web client is more responsive in compare of FLEX client and also performance is better, So the VMware tries to put the new features, components, solutions and etc in the HTML5 client.