Is concealment meaningfully better than destruction here?
I don't think so.
Perhaps the content should just be hidden and hard to access to reduce vitality but still allow people to view if they want? That wouldn’t satisfy everyone either though.
I agree with you, but to play devil's advocate, without public knowledge and access to the media directly, what incentive or pressure do terrorist organizations have? If the whole point of these acts is to instill fear, if they are not well-known the impact is limited. I can see that argument but don't have a good rebuttal.
Just like poker, terrorism isn't a thing that exists in a vacuum: it exists inside of, and in relation to, wider society.
The existence of the war crime allegations isn't being removed.
People horrified by the war crimes themselves provide the incentive/pressure. Photos/videos don't add to it, they just confirm the evidence.
I really don't think any victims' families are saying "oh well, because we can't see the videos anymore I guess we won't bother the government after all".
Governments generally only respond to public pressure or existential threats to their power and control and revenue, and little else.
Oh, but it is. The direct witnesses' accounts are minimized and ridiculed for lack of evidence, then the witnesses stop talking about it (normal ones; the crazy ones keep ranting about it forever) and the matter is totally gone within a generation. A historian might dig it up a few decades later to make a career and by that time it's too old to be relevant.
Consider these major events before they were part of accepted reality: NSA spying, coronavirus, the Holocaust, Uighur ethnic cleansing, Tuskegee syphilis experiments, etc.
Citizen outrage is the only lever we have to prevent governments from committing war crimes. If you support blocking access to the public, you're essentially saying these governments should be allowed to commit war crimes.
Another country's government could. War crimes can be prosecuted by any country (under universal jurisdiction), by the country on whose territory the crimes were committed (territorial jurisdiction), and also possibly if their citizens are among the victims (passive personality principle).
Even if in practice prosecution isn't feasible (such as due to inability to apprehend/extradite the defendants, a legal system which disallows trials in absentia, limited availability of evidence, etc), at least a foreign government could hold a formal investigation and publicise the results. Subpoenas aren't restricted to criminal cases, they can also be used for formal public inquiries (such as the concept of Royal Commissions found in Commonwealth countries, or the 9/11 Commission and Warren Commission in the US), for inquiries by the legislature (inquiries by committees of Congress/Parliament/etc), for intelligence collection, etc. It is also possible for a national government to subpoena data from private companies and then provide that data to an inquiry by the UN (or one of its agencies).
There is also the International Criminal Court (ICC). Iran is not a member state of the ICC, but the ICC could exercise jurisdiction over Iranian citizens in one of three ways: (1) if Iran were in the future to join the Court, or consent to its jurisdiction in a specific case; (2) if the Iranian citizens are accused of committing war crimes on the territory of an ICC member state; (3) if the UN Security Council made a referral. Now, in practice, Iran probably isn't going to join (or consent to a specific case), and China and/or Russia would probably use their Security Council veto to protect Iran, but method (2) might still work.
If some country's government, or the ICC, wants to subpoena data about Iranian war crimes from a social media platform, the social media platform is likely to comply (although there are all kinds of complex political and legal factors involved.)
For all we know, the content still exists on the social media platforms' servers, and access is just blocked to the public, but the platforms will happily provide it to the proper authorities in response to an appropriate request.
(Of course, I can't rule out the possibility they may have physically deleted some of it, or may do so eventually – I don't know what their data retention policies are – but I imagine they would be hesitant to permanently delete content that may plausibly be of future interest to regulators – this article contains zero information on whether they have actually physically deleted any of it or not)
Human Rights Watch is of course a purely private body, so unless they reach some special arrangement with the platforms (and platforms are sometimes willing to enter into those sorts of special arrangements), they are not going to have any more access to removed content than the general public has. But that doesn't mean that government authorities don't have more access than the general public has.