Would this feature be available as long as the change is not yet saved and while the editor is still open, or also when not saved and editor has been closed?

The feature is provided in the editor, so the editor must be opened to access the feature. I considered two scenarios:

A) The user opens an existing translation, modifies it and wants to revert the changes. When he deletes the text, an option to "revert to previous translation" appears next to the "copy from source" button.

B) The user edits an existing translation and closes the editor. Then, the message is marked as unsaved in the list. The user re-opens it again and the text area includes the "revert to previous translation" button.

In both cases, clicking the "revert to previous translation" fills the text area with the current translation and the editor behaves as if the translation was just opened (i.e., closing the editor will not make the "unsaved" label to appear in the message, and the save translation button would be disabled until the translation text is modified).

Patch:
https://gerrit.wikimedia.org/r/#/c/53758/

I discussed it with Pau and we decided that for now we'll show the "Discard" button immediately when the user starts typing and explore other scenarios later.