Why is Riot adding more true damage?

Because there are to many idiots thinking we are in a tank meta and that true damage is a counter to resistance stacking tanks (because armor/magic pen isn't the original counter right?). ANd in reality true damage is just a stupid design especially in large numbers, for example the little burn from red buff nice gimmick but nothing to hard. Fioras true damage? Way to much.

Cause they're idiots just throwing out random thoughtless changes to make things "new and exciting!"

Games are not short enough. They want FF@15 to be the option if your team didnt win by then.

True damage allows to get numbers not high enough to delete squishies but high enough to actually hurt tanks.

Originally (as in, back when the old balance team existed), true damage was a means of balancing a champion. By giving them true damage, they could determine how strong a champion would be in each stage of the game, since true damage is supposedly a rather fixed value, in both aspects that it doesn't fluctuate much whether said champ was fed or not, and also that it deals the same amount of damage to both tanks and squishies.

Enter the recent ("recent" aka recent years) balance team, who seem to have lost about 80% of that in translation, and rather than using true damage to restrict the fluctuation and properly balance a champion, it instead became the "strength" of said champion. [Read: Fiora rework having true damage massively scale on bonus AD, Camille's true damage massively scale on items, and it only gets worse from there.]

Cho'Gath's ult is actually a great example. Before, you could always know how much HP you have to be above in order to not get eaten alive by him, granted it wasn't a full AP Cho'Gath. Whether you're a tank or a squishy, you knew you had to be above ~300hp on levels 6-11, ~450 on 11-16 and ~600 on 16+. . . Enter Bonus HP scaling! Ha. Ha. Ha. Jokes on you. -Balance team.

Converting a % of your damage to true damage, as with Conqueror and PBE Infinity Edge, is a simplified way of saying "your damage gains penetration that increases in effectiveness the higher your target's resistances are".

It makes damage more consistent versus tanks while somewhat lessening the impact on squishier enemies.

I think they need to decrease damage across the board, stretch out combat, decrease all move speed, and remove true damage except smite, ignite, and red buff completely from the game. It's hard to differentiate one role from another when everyone has to be able to accomplish what they do within the 5 seconds it takes for a fight to begin and end. Of course everything ends up feeling like an assassin when you keep tuning all the champions to be able to be effective at what they do within a 5 second window. Adcs would deal consistent damage over a longer fight which would allow for repositioning, shields, and heals. Tanks could exist in combat and have a meaningful role as sources of cc and general disruption. Assassins could specialize in making sure that people who let themselves get too low don't escape combat instead of focusing on one shotting people from 100% health to 0. Supports could have items and skills focused on sustained team heals, shields, and buffs onstead of fat massive shields that make the games combat mechanics seem unsatisfying. Etc.

They saw that it worked well for the design goals of Conqueror and are throwing that same solution everywhere without considering why it worked well for Conqueror

Just had a game with a Yi (I was full armour HP Sion and we had a Rammus in the game), this Yi delt 68k to champions in damage, 35k of that was true damage. 35k of 68k. er.. okay. There was no point in me and the Rammus in the game existing, we were't a counter to him. Oh well, onto the next game.