I'm rather late to the party here, but I did notice that almost nobody has addressed the data that @LittleWhiteMouse presented in an analytical way.
As LWM presented, the amounts have significantly changed.
For CVs, T4 to T10, the percentage dropped an average of 4.2% across the board. Going from an average of 12.6% of damage received down to 8.4% of damage received. That's across the entire CV population and for all tiers. The largest difference was surprisingly enough at T4, with an 8.6% drop in DD torpedo damage received. The upper-tier CVs saw ~1% reduction in destroyer torpedo damage received over that time.
Battleships saw a similar drop (4.3% average) in received torpedo damage, again with the largest reduction in DD torpedo damage happening in the lower tiers. At T3, battleships saw their received torpedo damage reduced by almost half: from 20.8% to 10.9%. The higher tiers received a similar benefit, with an average reduction of 4.1%.
Cruisers, which have received primary access to radar, also saw reductions in received torpedo damage across the board. As a group they received 4.2% less torpedo damage between the two snapshots. T3 and T4 share the top spot for largest change, with each tier seeing a 6.4% reduction. The top tiers saw a 2.9% reduction, which is odd since the top-tier cruisers have the greatest volume of access to radar. T10 did see an appreciable change, with a 6.4% reduction in torpedo damage received. That is one-sixteenth of all damage received by all cruisers in the game.
As a class, Destroyers saw the smallest reduction in received torpedo damage at 2.1%. T5 saw the greatest reduction (4.1%), while the top tiers got a 1.5% reduction. This isn't terribly surprising, as destroyers are the fastest and most agile ship type in the game, and have the easiest time dodging a fish.
LWM states that these two graphs date from end of 2015 (I'm guessing December) and September of 2017 respectively. Not quite two years between them, and we've seen a drop for every ship type vis-á-vis torpedo damage received. Even at its height, approximately one-fifth of battleship damage in two tiers (T3 & T10) came from torpedoes. One-fifth, and it was christened "Torpedo Soup"? The worst that cruisers had it at the time was 15%, at T3. It is all borne out in the numbers listed, go look for yourself and you'll see it.
CVs saw a 12.1% increase in AP damage and a 0.2% increase in HE damage. That's opposite a 4.2% decrease in torpedo damage.
Battleships saw a 1.5% increase in AP damage and a 2.4% increase in HE damage. Opposite a 4.1% decrease in torpedo damage.
Cruisers: 12.7% increase in AP damage, and a 4.4% decrease in HE damage. Opposite a 4.2% decrease in torpedo damage.
Destroyers: 10.2% increase in AP, and an 8.4% decrease in HE damage. Opposite a 2.1% decrease in torpedo damage.
That is across all tiers, mind you. The top tiers saw some odd changes over this time period:
Top-Tier CVs received a 20.1% increase in AP damage and a 6.4% increase in HE damage. Opposite a 1.1% decrease in torpedo damage.
Top-Tier Battleships received a 1.5% increase in AP damage and a 2.4% increase in HE damage. Opposite a 4.1% decrease in torpedo damage.
Top-Tier Cruisers received a 9% increase in AP damage and a 4.7% decrease in HE damage. Opposite a 2.9% decrease in torpedo damage.
Top-Tier Destroyers received an 11% increase in AP damage and a 9.5% decrease in HE damage. Opposite a 1.5% decrease in torpedo damage.
I'm not entirely certain what all these numbers mean as a whole. I did leave out a few categories for the sake of keeping this simple. The pattern that I am observing is an across-the-board decrease in torpedo damage, which has hurt a group of players that already didn't have the easiest time as gameplay went. Every top-tier group (T8-T10) across the game has seen less torpedo damage, from what wasn't exactly the most competitive set of numbers. While damage has been compensated for in other areas -- particularly AP damage, which increased for literally everyone -- the crux of a destroyer lies in her torpedo tubes.
I don't know if anyone else remembers, but the 'top ships of the month' thing has yet to list a destroyer in the top-5 for most-played in a month. As I recall, there isn't a single destroyer in the top ten ships for most-played when measured in terms of games played. Destroyers are a critical part of the game, and it seems like they aren't really popular. The longer this goes on without any tangible changes, destroyer players will not want to keep playing. This is alienating players, as these forums have shown.
As for why they aren't popular, I think I have a pretty good idea for why. Destroyer play is "short, brutish, and nasty" as a matter of course, even without radar in the game. Add in radar and that life often gets much shorter. While it is very true that evading radar is a required skill for any DD captain, radar does have some problems that need to be addressed. The "spot through islands" bug is my chief complaint, though there are a few other problems with it as well.
As for the conversation about "hard counters", some of you seem to have forgotten another hard counter to smoke: Hydro. Almost all the high-tier cruisers and a lot of mid-to-high-tier destroyers have access to Hydro. Anyone who plays German destroyers knows this quite well: Bring a friend or two in a cruiser (CLs are great for this), watch for a cap to start turning, run in, fire up the hydro, and feed an unsuspecting DD to the cruiser(s) following you. I've done it before, it is a legitimate tactical method.
The element that has been missing from that conversation so far lies in what destroyers are supposed to do: cap and scout, sink ships when practical. Anyone in a cruiser with radar can see that a cap is being taken, run over, use the consumable, and that destroyer has to run until the radar runs out. Best case, he takes no damage and has to turn around to try again for the cap (done that as well). Worst case, he loses a large chunk of his health and has to walk on eggshells for the rest of the game. How many times have you spotted a DD in a cap circle, he goes to flee, drops his smoke to cover his retreat, and he gets lit up by radar? What happened next? Did the ship make it out? I will bet that the destroyer didn't make it much farther, or was seriously damaged from the ordeal.
If you don't play destroyers, I would like it if you tried them out. Get to T7 and try capping when there are three to five radar cruisers on the opposing team (double points if you are bottom-tier). You will spend a good chunk of the game running from that radar and trying to cap or scout. Good luck sinking anything with your torpedoes, as most battleship players appear to also know which cruisers have radar and will stay firmly behind the safety of radar cover. I'm willing to bet that the use of radar as a shield for battleships has also contributed to the passive meta. Throw in the hit percentage numbers for destroyer torpedoes (hundreds fired in a single match, maybe a dozen will hit), and you'll see that something is amiss.
I will not advocate for radar to be done away with. It is in the game, we have to live with it. It does need some adjustment, and I would like it if WG did not add radar onto any more ships until that adjustment is completed. Just like the RN CV line, I don't think anyone is expecting it to come out before the other two lines have been fixed. Let's take a tactical pause, fix what needs fixing, and then move on.