while i sort of agree that true damage is imperative on certain champions to slightly give them an advantage over other champions, i do not agree that this stat should exceed 10%. prime example is yi & velkoz whose kits allow them to utilize true damage but typically exceed 10%. the purpose of true damage is to shred through tankier, more resistant champions however the two champions listed bypass that concept because their ratios reflect their own statistics and not the enemy's HP value or resistance. this means that their damage is a flat value rather than proportional to enemy stats.
i suppose that makes the original issue into two large problems. their kits exceed the 10% true damage limit (limit established in my opinion) and that the true damage pumped out only reflects their own ratios.
master yi's E is maxed last because it isn't as pertinent to level compared to the rest of his kit but the ability is a stat cannon paired with his passive and guinsoos. essentially, he doesnt _need_ to rank this up at all since the flat damage is hardly noticeable, especially on tankier champions. the % ratio (35% bonus AD) conversion into true damage at rank 1 is insane. this champion is supposed to scale, so why give him 5 seconds to abuse that crazy ratio? If he obtains 100 bonus AD, the ability for 5 seconds does 193 true damage without thinking about any additional damage that comes from his passive or guinsoos. if this champion is supposed to scale then that part of the ability has to scale instead of starting off at 35%.
same with velkoz whose passive flat stats are hardly noticeable from level to level but maintains a 50% bonus AP ratio. this is why this champion hits so fucking hard off of one item given he hits 3 abilities or just uses his ult during a team fight. the same reason is why he can go botlane and annihilate the botlane with little ap.
i'm really tired of writing more and more about this. my main point is that there needs to be a hard limit for true damage but enough for champions to do marginally better if they're played correctly.