4.1 should be along soon, we’re finalising our timeline for that now. Everyone will be able to try 4.1.0-rc1 by the end of July (4.1.0-beta is already available too via Docker).
Just to clarity, the feature wasn’t removed, the embedded HTML widget is available from the toolbox as before, and any existing widget templates which had “Override Template” selected are available to continue editing. We did remove the user interface customisation of HTML/CSS templates (“Override Template checkbox”) and replaced it with the new Elements system for a better user experience. You can create almost anything you used to be able to create with HTML/CSS using elements instead.
The ability to customise templates is still available via developer tools; which is the functionality we’re exposing via a developer page in the CMS UI in 4.1. A user with access to the developer tab will be able to copy a template and supply HTML/CSS/JavaScript in a structured way, which will be applied to any Layout where that template is in use.