If a user edits a project description, the change is logged in the project history, but no diff is available. In comparison, diffs are available when editing tasks or comments.
In Phabricator it is usual to find projects that any registered user Can Edit, since this is the default behavior. There is no reason to restrict this policy, because just like in a wiki, most edits contribute to better texts. However, this is also an exposure to vandalism. If someone vandalizes the description of a project, there is no diff to restore. This might be a small problem for one line project descriptions, but as instances mature, it is possible that some projects include documentation of such projects, processes to use a tag, etc.
This task comes from this request: https://phabricator.wikimedia.org/T75735