There's a problem with your shackle theory. There is a good chance that when a boat gets shifted off to the side by the end or current or both, particularly if it's a combination rode with only a bit of chain at the anchor end, the tension on the rode could be enough to lift the shackle, pull it over the top, and get it sliding down the slot to potentially unset the anchor. This likelihood would be reduced to a degree by using an all-chain rode with more weight to hold it down, but it's still a possibility
But as far as I'm concerned the whole concept of a slotted shank is a Very Bad Idea
except in the case of deliberate anchoring in places with foul bottoms where the anchoring is only temporary. The prime example of this is sportfishermen who often anchor for bottom fishing or mooching in or near kelp beds or at the edges of or over reefs or in rocky areas where fouling an anchor is a very real possibility. Often these people fish for awhile, pick up and move, fish for awhile, and so on. An anchor that's easy to free when it gets stuck is a real benefit, and is the reason for Rocna making a slotted shank version of its anchor in the smaller "fisherman" sizes.
Manson did a smart thing by putting a shacke hole in their anchor shank along with the slot. Every Manson Supreme I've seen-- which has only been a few; they are apparently not very popular in this area-- has been shacked to the hole, not the slot.
Which goes to show that while the owner might not have been smart enough to buy a Rocna or order a Sarca, he or she is at least smart enough not to shackle to that stupid slot.
Of course one has no choice with a Sarca but it's easy enough to torque a nut, bolt, and lock washers down in the slot to ensure that the shackle can't slide. If we lost our first-generation Rocna for some reason we'd be inclined to order a Sarca. But we would either ask the manufacturer, or have it done here, to weld the slot closed at the attach end so we had just a hole to attach the rode to. We believe in reducing the risk of anchoring problems to the absolute minimum, and the slot is an unnecessary risk in our view.