Mages have been doing True Damage to squishies with just boots literally for years.
Hey, since people might not know, I figured I would point out that Mages have been doing True damage to squishies for years now with absolutely nothing but Runes, Masteries, and Boots.
How?
Well lets take a look at something that you might not know. As it is currently, half of the champions in LoL have a base MR of 30 at level 1 and get ZERO MR/Level. This leaves them with 30 MR from levels 1 to 18 unless they go out of the way to build some.
So lets take a look at the numbers.
Starting with 30 MR:
-
minus
= 15 -
minus Magic Pen Marks = 7.2
-
minus Precision (Mastery) = -1.2
So with just runes, masteries, and 1100G boots, Mages have been capable of dealing true damage to squishies for years now.
For comparison, The LOWEST base armor at 18 is Orianna with 68 Armor. With the exceptions of Thresh who has his passive and Gnar who has different values in Mini/Mega.
So if we're going to be fair about this, True Damage to squishies has existed for a very long time.
Some food for thought.
for example

and none for
? I see how it is
works on TOTAL magic resist, so its still very effective vs squishies
.
as their first item but for
and maybe
, everyone else won't even think about grabbing it until it's their 3/4th item and that is even if they need it at that point.
etc
was a core support item built almost ever game that gave an aura of MR to all allies (I think the component had it, Locket had it for sure). So they wouldn't take true damage because allies gave them MR.
) which introduced additional AP ratios to champion's kits which required the reduction of AP across the board because of the ridiculous damage output (not to mention masteries).
(and ADCs could buy it too) and suddenly become immune to being bursted. Fun for the mage.
wasn't enough since if you used the stasis for pretty much anything other than a Zed ult, you're just sitting there as a free target for their entire kit when you come out (in lane, late game it's a whole different story).