Before going in to why tanks are currently some of the worst champs in solo queue right now, I want to make some things clear. This is mostly from my observation from playing almost strictly tanks top for the past 4 season and watching competitive league for a while.
Tanks have been the literally and figurative punching bags of league for a while now. The community has a great distaste for them and there is good reason for that. In competitive, having tanks in the game makes games way to long and extremely boring to watch. In solo queue, they seem like unstoppable monsters that your ADC love to walk and suicide into. There are definite reasons to hate tanks and I am not saying that they should come back or be buffed right now or anything like that, I actually think top lane as a whole is pretty good with good diversity and only a few problem champs. I am going to explain why they feel so terrible right now, what led to this low, and what possible changes could be if Riot really wants to bring them back.
I believe it all started with just general champ nerfs, across the board, tanks have been nerfed in almost every aspect of their characters. Just to list a few: Nautilus with his mana and E nerfs, Maokai with his constant changes to his kit and his stats, Gragas with E and passive, Ornn with his W and ult (though his kit is just too good to nerf completely), Shen with his Q nerfs, Sion E and Q nerfs, and the list continues with other off tanks champs like Poppy and Ekko and Fizz.
The next big hit they had was the item changes. Going for having more health on items to having more resistances really hurt tanks as they did not have the ability to survive both types of damage from two carries. It was also worse since resistances start to benefit less the more you get and stacking two items of the same kind like a sunfires and omen was much worse than it was before. Changes to items like Doran's ring also hurt almost all magic damage tanks like tank Galio and Maokai forcing them to not only lose their ability to wave clear, but also fight back in trades.
Next there are champion changes/buffs. As more and more champs get reworked and added, there have been more and more bad match-ups for tanks to play in the top lane, some notable champ are Illaoi, Darius, and Fiora. They are all great at punishing tanks and matchups for tanks to survive without losing too much becomes few to none. There are no more winning matchups for tanks, and the matchups where you might go even have almost disappeared as well.
This goes hand in hand with the biggest changes of recent times which is runes. People think that conqueror is the biggest problem for tanks but there are problems that hurt them even more. The lack of a good keystone is one, tanks are almost forced to go into grasp as their rune because of the nerfs to aftershock and spellbook has left tanks with a sub par rune and it even forces some tanks like Sion to go into Glacial Augment or Aery on Poppy. One other big rune that might not be so problematic right now is Klepto, it put tanks so far behind as they are just a walking sack of gold.
The last thing that brought tanks to this position is the Community. We have brought upon so much change to the game whether it is good or bad and tanks is just one case of this. Other examples include: Recent ADC nerfs, Jungle nerfs, and even things like the changing of gold bounties.
Tanks are in a bad place because a combination of many factors that have compiled over the years; this is just the most recent low and will most likely not be the only time tanks are in a bad spot.
Now to look towards the future, tanks will 100% come back, there is always a tank meta, and champions come and go through the years. To make tanks more healthier for solo queue, I believe that item changes are the best route of action. In more recent times, tanks have done way to much damage early on and in mid game while still being "unkillable". I think that items changes would be best STRICTLY FOR SOLO QUEUE as it will allow tanks to ramp into items and stay long enough in fights. Tanks are not late game champs, they are mid game champs due to their cheaper items and survivability, once champs get resistances and %pen, tanks fall off much harder. Improving tank itemization will enable them to move away from the brainless banner and push and focus more on being a tank that soaks damage. Tank damage is already really poor right now and keeping it lower is the best to ensure that bruisers still have a spot. This would greatly hurt competitive but the game can not always be balanced around competative and there will be more ways to counteract these possible changes.
Nevertheless, tanks should take their break from the meta and come back with something new and fresh that can rebalance them to have a meaningful role in the game, especially something that is not as brainless as the banner push meta.