Patch 7.4 (CRIT ADCS + LW CHANGES)
So as of now, lethality adcs are the meta, such as jhin, varus, mf etc and normal crit adcs aren't favoured heavily. Patch 7.4 nerfed lethality slightly (with supposedly more changes to come in 7.5) and was also supposed to give crit adcs a boost in the early game so they don't feel like they're "fighting a fire with a squirt gun".
Now one of the primary changes, other than the nerfs to lethality, that was meant to help these marksmen's impact in the game was the last whisper "buff". Riot buffed LW from 30% armor pen to 45% armor pen, but reduced the damage on it by 15. When I first saw the changes, I also thought "rito finally did something to help crit adcs make a larger impact in the early-mid game omg". Sadly, this turns out to be not true.
A redditor recently posted in a thread concerning the new patch a series of calculations based on the damage and bonus armor required for the new 7.4 LW to be better than the old 7.3 one. The google doc link is attached. To summarize the data, a crit marksman would need to hit an average of 180 damage per auto and the enemy would have to have at least 150 bonus armor for the new LW to deal more damage. 180 damage is standard or a little less for the average mid game adc (IE, shiv, lw). The requirement for the change to be beneficial to crit adcs is that a tank would need 150 bonus armor. This is a little ridiculous and you would need multiple armor items to get there. For example, a top lane nautilus with a randuins omen and a sunfire cape would only have 110 bonus armor. The only way to achieve approximately 150 bonus armor is with extreme situations. Eg. if your team was all ad then the enemy tank can just stack a thornmail and randuins for 160 bonus armor. However, in most cases, randuin's omen, sunfire cape, and deadman's plate are the ones tanks opt for in this meta because of the hp it gives (unless you're playing rammus top).
If we were to take a look at the second half of the data, the damage is increased to 450 (mid to late game adc) and even then, the opposing team would require 150 bonus armor for LW to be effective. So the new LW relies almost solely on the amount of armor the enemy team stacks.
The worst part isn't about how the new LW generally doesn't help adcs do better against tanks. The worst part is the nerf to the damage dealt to non-tanks who don't build armor. Normally, a well formulated team composition has approximately 2 tanks, which means the other 3 are squishier targets who don't stack armor. There is very little incentive now to buy LW in the midgame because of the huge damage nerfs to nontanky targets. Eg. the data shows that you're dealing 7% less damage against a target with only 9 bonus armor in the current patch with LW. This means that the new LW is an insanely unfavourable buy for the mid game (where you're hitting more than tanks).
This isn't helping early-mid game crit adcs at all despite patch 7.4 being geared towards them for the mid-early game. The warlords nerf weakened most crit adcs in lane and the fervour buff was barely one that helps (6-8 seconds?). The new LW is definitely not geared towards the early-mid game and adcs have less incentive to buy it now. In addition to these early-mid game nerfs, the late game is definitely not any better either. The nerf on LDR reduces counterplay and damage against half tanks such as Yi or Jax who build a mixture of damage and tank. Ultimately, patch 7.4 was very disappointing for me (I don't play lethality adcs) and I hope Riot addresses the new LW change.
tl;dr: Riot promised to make crit adcs better, but instead made them worse indirectly by nerfing LW and a few other things.
Here's the reddit link: https://www.reddit.com/r/leagueoflegends/comments/5vkm33/patch_74_notes/de30wiy/
and the whole thread: https://www.reddit.com/r/leagueoflegends/comments/5vkm33/patch_74_notes/