These got a lot of attention a while back. But their use case is limited because they're really short-ranged. These things go all out soon after leaving the tube, make a hell of a lot of noise, and are good only really close in. They're sort of a last resort weapon, a snapshot provided that the enemy shooting at you is even within range.
A modern US Mark 48 can swim out quietly under wire guidance and emit no noise or sensor emissions, steered on an intercept course, then revved up to top speed once it's in close. A torpedo like the Shkval can't close the distance, and that's assuming the enemy even knows where they're being shot from. Just because a Mark 48 suddenly goes active on your stern doesn't mean it was fired from there.
According to the wiki article, this was for intercepting incoming torpedoes. I suppose that could work, though again, the noise I'd imagine would be troublesome. It might mask for you a bit, but your attacker would know where this thing was shot from. Meanwhile, your own sonar is only going to hear this thing, meaning you've lost a solution on the guy shooting you, leaving him to set up again.
I highly suspect the reason we haven't seen an equivalent is because the tactical doctrine doesn't call for one. My armchair two-cents.
My personal theory is that a big use case for Shkval is in the littoral and shallow waters of the Baltic Sea where there is fantastic opportunity to play hide and seek. Especially sub to sub. I think the Soviet command expected enemy subs to successfully sneak up on them and then have something up their sleeve to raise hell right back.
For that kind of case, wouldn't it make more sense to keep the torpedoes deployed all the time at 100 meters or so from the main sub, so that they can counterattack after it gets destroyed? It could drag them behind it on cables.
