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I kind of doubt that there is a standard for it, at least. You might get away with using some marker that isn't used anywhere else, like ###, to indicate a changed passage, but I'm not sure if that...
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#4: Attribution notice removed
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/6865 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I kind of doubt that there is a _standard_ for it, at least. You might get away with using some marker that isn't used anywhere else, like `###`, to indicate a changed passage, but I'm not sure if that really qualifies as markup. In any case, it would have to be an agreement between the people involved which marker(s) to use and what they mean. Something like this: Tracked changes look like ###this[WAS: these]###. That said, depending on the technical inclination of the author(s) and editor(s), you might get away with something like setting up a GitHub account and uploading the text file there. Source control systems tend to be somewhat more geared toward programmers, but they solve your problem quite nicely: they allow multiple persons to work on a single text file (even simultaneously), tracking changes over time, displaying the differences between arbitrary versions, going back and forward between versions, and selectively rolling back any changes made. It may take a little getting used to for someone who isn't technically inclined, but if you are serious about doing this with only plain text files, it's a reasonably easy way forward once you get over the initial learning curve. And for writing, there's likely no need to deal with the more complex issues such as branching/merging and such; a linear history will likely work well enough.