Good news! Visual diffs for history pages are available as a beta feature. Try it out. I'd love your feedback.
Also, visual diffs in the visual and 2017 wikitext editors are the default now, and are no longer in beta. :-)
I just checked now, and visual diffs are definitely the default on the Czech Wikipedia if you're using the visual editor or 2017 wikitext editor. You can test what the default is by using an incognito or private browsing window. Your previous choice of diff mode is remembered though, so if you switched back to wikitext at some point, then it'll keep using that until you go back to visual. :-)
Ohh, sorry for me, I see, there is another problem:
If you go to the preferences page (on Czech Wikipedia), there is a beta feature where you can turn on Visual diffs, but the description and the "more info" link is rather confusing and writes about Visual diffs in VE. See the description on preferences page: https://cs.wikipedia.org/wiki/Speci%C3%A1ln%C3%AD:Nastaven%C3%AD#mw-prefsection-betafeatures and the "more info" link target: https://www.mediawiki.org/wiki/VisualEditor/Diffs
This is really confusing. I thought if I turn this betafeature on, it will turn on Visual diffs in VE, but I tried and was really surprised I got Visual diffs in history page! This should definitelly be fixed