Few sights used to put a chill down a player's spine than the sight of a Shadowsword. With a massive range, thick armour and a cannon singularly built to slay Titans, it served as the ultimate tank destroyer. If it fired its gun anything caught in its path was simply gone, not merely blown up, shredded or even melted, but completely atomized. This was supposed to take the Imperial Guard concept of a big gun on a big tank to its logical extreme, but today you'll rarely find them being fielded. Save for the odd big game or one kept aside as a centerpiece, these machines are regarded as effectively useless in the face of current armies. So what the hell happened since their introduction to make such a powerful machine so badly sidelined?
Perhaps the single most clear answer is that Warhammer 40,000 underwent a series of massive changes in a short amount of time. After over two decades of playing it relatively safe, suddenly the game exploded with new ideas; first allowing Super Heavy class vehicles into Apocalypse and then standard games, then adding flyers as a core part of the game, then building up walkers into mini-Titans and even reworking the psychic phase from scratch. While Warhammer has always been willing to experiment with newer, pricier ideas to keep the ball rolling, this went into complete overdrive. Rather than a couple of changes per edition we started to see things evolving further and further with every couple of years, each and every one of which drastically reshaping the game.
The constant evolution of tabletop warfare in Warhammer allowed certain concepts to become far broader and offered players far more versatile ways of tackling a foe. Psykers, for example, have quickly gone from a nice bonus to a one man annihilation unit, capable of slaying entire squads at a time. Not only that though, but many have taken the Farseer approach of opening up ways of reshaping the battle or directly buffing their allies. True, these have all been aspects which have existed for a long time, but they have been exaggerated their scope broadened. The recent Angels of Death introduced a couple of solid examples of this such as Geokinesis, which offers everything from the ability to rearrange the battlefield to healing allies. The concepts themselves kept evolving and developing, offering more ways to approach things.
We saw the same thing happen with flyers as well. Much like the Super Heavy vehicles, they were originally introduced via Forgeworld serving as little more than tank hunters. Effective ones to be sure, ones which could barely be hit or truly nailed down, but nevertheless they largely retained one single use beyond the likes of Thunderhawks. However, this too evolved quite quickly once they were added into standard tabletop rules. After all, they made their splash with something which was aptly nicknamed "The Flying Land Raider" among the few positive names. So, rather than a couple of guns and a few missiles, you now had vehicle with a wall of guns capable of slaying infantry, vehicles, flyers and also dropping off troops while they're at it. Oh, not just any troops though, but just about anything from Terminators to Death Company forces, often backed by a Dreadnought. Ah, and if that weren't enough three of these could deposit the bulk of an army into the enemy's deployment zone on the second turn if they were lucky enough. Add in the more bizarre stuff like some of the more unusual Chaos Dragons or Necron Death Croissants, and all of a sudden it becomes a much, much more versatile option for an army. In many regards, almost a necessary one.
By comparison, the Shadowswords have changed very little at all. Their role remains ultimately the same, just rolling in to blow up tanks and entrenched targets, and that's about it. While they have just as much range and armour as they did before, they retain all their weaknesses: An extremely slow speed, a lack of additional weaponry for their cost and the ability to be impeded by terrain with ease. You might expect that they would have some additional benefit these days with invulnerable saves being so commonplace and the ability for an army to literally buy bunkers, but that's sadly not the case. All the damage it can cause to those targets is usually handled just as well by aircraft; which have the added bonus of overcoming line of sight issues and have methods to overcome a lack of a Strength D weapon. It ironically fits in with the old Wizards vs. Barbarians argument many so often bring up in RPG criticisms. The former develop a hundred new skills and abilities - including those which outdo the latter at their own game - and their counterparts are still stuck with only one skill to their name.
However, the growing influence and domination of easier alternatives over the game isn't the only difficult factor. You also have power creep to contend with. Now, before you roll your eyes at this, let's just stick to the basic facts - When the Shadowsword was introduced, its whole gimmick surrounded its Strength D weapon. While exactly not one of a kind, something like this was rarely seen and practically unheard of before they rolled onto the scene. This allowed them to wipe away old favourites like the Land Raider, Necron Monolith and tough targets with ease, making them unique. Like the other examples they made one hell of a splash, but then the rest of the game caught up with them. People might have rolled their eyes at a Space Wolf character being able to throw a hammer with the force of a railgun round, but things then just became obscene. Armies started abruptly gaining one or two Strength D weapons per codex via relics or standard weapons, and some started to even become standard equipment. Rather infamously, the Wraithguard now carry rifles which hit with the same strength as a Titan killing weapon, and the Salamanders suddenly gained a relic out of the blue in their most recent update.
So, not only had the Shadowsword been dethroned by other units, but its semi-unique gimmick has become practically commonplace. There are cheaper, easier and sometimes downright better alternatives now, and even a weapon limited to close combat isn't all that much of a hindrance these days. Given all of that, the game reshaped itself and was reworked until a single massive target largely reliant upon a single gun simply can't justify its points. As before, some people will find their uses for them either out of a desire to stand out or be that little bit more different, but those after a truly competitive force will rarely delve into them.
Ironically, this evolution actually reflects a shift in tactics found in real life during the Second World War. There are many examples of where people found that slow moving heavy armour was found to be lacking, but one of the most apt examples originates from the very end of the war. Alongside the continued failure of Germany's massive super tanks, Britain and America took a stab at building such vehicles for themselves. The end result was the T28 and the A39, only the latter of which truly reached combat. While they retained the armour generals desired, they had the firepower and the latter example was found to be mechanically reliable, they quickly became a third wheel in the face of more mobile warfare. Infantry portable anti-tank weaponry was rapidly advancing now, and aircraft themselves had entered the jet age, with several examples already carrying heavier payloads by the year. Before-long, the titans which many designers hoped would become icons had become all but obsolete.
By this point - short of giving it wings, a jet engine or an extra major artillery piece - the only way the Shadowsword might bounce back is to boost its strength. Even upgrading it to a Strength Double-D would unfortunately run the same risk as last time, with designers feeling the need to increase other weapons to their absolute uppermost limit. So, outside the realm of unique Special Rules, there's sadly not much to really be done. Perhaps we'll see a few inventive ideas involving the tanks later down the line, but it would take a serious reworking of the vehicle to return the Shadowsword to its fearsome glory.