This feature request doesn't describe a problem, see Describing Root Problems. Specifically, "feature doesn't exist or might cause an issue" doesn't give us any indication how people are having issues using Phabricator.

I think the problem "People can hide tasks from admins and staffs, and you need to investigate much work to find this tasks, and at a worst case scenario people can do illegal thinks at that task, which is not visible for you" is a root problem, because you can't control all tasks at a public instance without forbid the people to mark tasks as not public visible. So you can't give people a chance to report security issues, or issues/requests with private data. This is an existing problem. For example Wikimedia wrote a 3rd party extension for this problem, which lets herald change the policys, without let people adjust the policys at their own (they can choose at a dropdown). So the problem exists, and it can easily solved, by the proposed solution.

Has that actually happened? Our expectation is non experienced users who don't need certain fields should just use a custom form.

Is this an actual problem you've experienced, or purely hypothetical?

What specific harm have you experienced as a result of users hiding things from administrators?

The worst case scenario was not real, but some time ago I was admin at a instance with public registration. We disabled the possibilty to let users adjust the task policys, after two users abused this to hide important tasks from developers. Sure, we restored this tasks by bin/policy, but we had first to find out, which numbers the tasks had, so we had to insert time in a problem, that can be easily fixed. After this, we disabled the ability to change tasks poliycs for unstrusted users. The problem now, is, that if new users have tasks with confidential informations, they need to contact us at another way, which means more work too, but makes existing tasks more secure. My proposal above has the advantage, that they can report confidetal data, if they change the space, but they had no change to hide data from us, and mass abuse can reverted by a bulk edit.

after two users abused this to hide important tasks from developers

To clarify, these users were hiding tasks that other users had reported?

You can prevent this with policies, and this install is configured to do so (for example, you can not hide any of the recently-reported tasks that you didn't report, since you don't have permission to edit them).