I don't think that undo has much to do with the type of app, but rather with its design. So mobile, web and desktop apps should be able to offer similar functio

I don't think that undo has much to do with the type of app, but rather with its design. So mobile, web and desktop apps should be able to offer similar functionality. At least for delete many sites are using a deleted flag anyway, which should be easily reversable. Getting back even part of the content might be worthwile. In your example, if user 1 doesn't see the edits of user 2 there's a bigger problem. Undo would be liniar.