There is no risk - and nothing to disclose in the 10-K - because the ability to control whether or not Wizards/Hasbro gets sued lies entirely in their hands. So long as Wizards abides by the Reserved List policy, there is 0% chance they will be sued for violating the terms of that policy. Just like there is no duty for companies to warn in their 10-K that there would be a risk of liability if the the company suddenly chose to engage in false advertising, there is no duty for Hasbro to "disclose" a "risk" that they have complete control over.
Given the presence of the B/R list and the public knowledge that any card may be banned or restricted in any format, there is no reasonable reliance interest in keeping certain cards from being restricted. In fact, given that there are constant new printings of cards, you couldn't maintain a cause of action if they printed Mishra's Double Workshop that taps for 6 - you know there are going to be new printings of new cards that may obsolete your reserved list cards. Plenty of reserved list cards have been obsoleted. But WOTC has explicitly stated "Reserved cards will never be printed again in a functionally identical form." That statement is aimed at consumers of those cards, WOTC clearly knows persons may rely on those statements, yadda yadda. Whether it would stand in court - and what the damages would be, given the limited print runs of some of those early cards - is an open question, but I think the legal risk is definitely a deterrent to WOTC. Of course, they may also want people to feel confident that they can buy into their game and retain an interest in the game that will have some sort of value, even if the value will fluctuate, and going back on the reserved list policy for a quick buck might harm that perception. But I'm not sure that gives us any foundation to predict what Wizards will do vis-a-vis the B/R list, much less what it should do.
Also, for the record, I disagree with @smmenen's policy preference/prime directive - I would prefer to keep as many old cards unrestricted as possible, even if that means cutting out some new toys. But I don't think that Wizards sticking to the reserved list policy means that Wizards (1) has some principle of "don't harm consumer's wallets" that it consistently sticks to or (2) provides a useful analogy for theorycrafting B/R policy.
And one more thought - when it comes to discussing B/R policy and the state of Vintage generally, I don't think there's anything wrong with approaching those questions with policy preferences and focuses that differ from WOTC or from the community as a whole. Given how erratic WOTC has been in administering the B/R list, an off-the-wall approach may in fact align better with what it ends up doing than whatever TMD's general consensus may be. I just think it's useful to make your preferences and biases as explicit as possible to limit the possibility of misunderstanding (and the frustration of arguing with people on the internet).
@ten-ten It won't happen until maybe it does happen. It's open design space. Literally the first thing they did upon abolishing the planewalker uniqueness rule is to create a planeswalker that can duplicate himself. At some point, it's likely that they will create a low-power planeswalker that has no legendary subtype at all. "All" means "all" only when it doesn't mean "all except for one or two," but Wizards often means the latter.
Yes, that is absolutely correct - these cards are not part of anyone's collection, they are only accessible in the preconstructed deck event, and they will not be accessible to anyone after that event ends.
However, as @Aelien pointed out, these cards are not only functional (granted, none of them are super complex) but several of them are also animated, including at least Mox Pearl, Mox Sapphire, Sol Ring, and Black Lotus (not sure about Ancestral Recall). Given that the preconstructed deck event is a short-term promotional event, it seems odd that they would devote animation efforts to these cards unless they have at least an inkling that they may bring them back for a more sustained outing at some point.