In this context, I would like to draw your attention to a function of DokuWiki that you may not be aware of and which at least partially covers the functionality of the plugin requested here:
Every time you click on the preview button, the current content of the page is cached on the server. This means that it is possible to access this cached version. To do this, you must exit the edit mode, of course without saving beforehand. (To be on the safe side, I often close the entire browser window.) Afterwards or when the page is completely reopened, the Edit button changes to Draft-Edit button (or perhaps a similar name). This can also be used to deal with problems such as disconnections.
And if you make a change to the original, completely unchanged version of the page, you will be informed that a draft version is available.
See also https://www.dokuwiki.org/draft
- Michael Sy.