This is what I see when I visit the mobile page (https://en.m.wikipedia.org/wiki/Special:MobileDiff/666058337):

Is this not what we want?

Per Baha's comment, is this task to add a message like on desktop? "You cannot view this diff because one or both of the revisions have been removed from the public archives. Details can be found in the deletion log for this page."

The behaviour between https://en.m.wikipedia.org/wiki/Special:MobileDiff/666058337 and https://en.wikipedia.org/w/index.php?title=Wikipedia&diff=prev&oldid=666057460 is different. With a little investigation it should be clear why (they share the same code)

Specifically the text "You cannot view this diff because one or both of the revisions have been removed from the public archives. Details can be found in the deletion log for this page. " should be shown

Notes for me:

• Desktop diff: https://en.wikipedia.org/w/index.php?title=Wikipedia&diff=666058337&oldid=666058325
• Mobile diff:
  • (Explicit works) https://en.m.wikipedia.org/wiki/Special:MobileDiff/666058337...666058325
  • (Explicit broken the other way around) https://en.m.wikipedia.org/wiki/Special:MobileDiff/666058325...666058337
  • (Previous broken) https://en.m.wikipedia.org/wiki/Special:MobileDiff/666058337

Change 405346 had a related patch set uploaded (by Niedzielski; owner: Sniedzielski):
[mediawiki/extensions/MobileFrontend@master] Fix: check old revision for deletion when diffing

https://gerrit.wikimedia.org/r/405346

Some repro steps:

1. Create a new page. On my instance, this is rev 7. (Rev 8 is a File page for an image upload.)
2. Edit the page. On my instance, this is rev 9.
3. Delete the page (requires permission). On my instance, this is rev 10.
4. Undelete the page (requires permission). On my instance, this is rev 11.
5. Visit the two revisions in each state:
   • http://localhost:8080/wiki/Special:MobileDiff/10...9 (deletion)
   • http://localhost:8080/wiki/Special:MobileDiff/9...10 (deletion)
   • http://localhost:8080/wiki/Special:MobileDiff/10 (deletion)
   • http://localhost:8080/wiki/Special:MobileDiff/9 (edit because implicitly diffing against 7)
   • http://localhost:8080/wiki/Special:MobileDiff/11...10 (deletion)
   • http://localhost:8080/wiki/Special:MobileDiff/10...11 (deletion)
   • http://localhost:8080/wiki/Special:MobileDiff/11 (deletion because implicitly diffing against 10)
6. Check more "normal" diffs like the creation diff have no regressions: http://localhost:8080/wiki/Special:MobileDiff/7.

Change 405346 merged by jenkins-bot:
[mediawiki/extensions/MobileFrontend@master] Fix: check old revision for deletion when diffing

https://gerrit.wikimedia.org/r/405346

This is working for me on the target URL in the description. Here some shots. I did notice the message has some weird text wrapping on phone width devices (@Nirzar).

Yes,the word wrapping needs to be present for diffs which may have lines, but it shouldn't break the word for warning messages. I'm guessing a css fix for this should be trivial.

bug opened here: T187173: Mobile diff warnings do not wrap properly