These are all changes to how and how much damage is dealt by turrets since season 1:
6.9:
Outer & inner turret damage
Turret damage increases by 37.5% for the second and third shots against champions, then 25% for the following two against a single champion (resets on target change) ⇒ 40% for the first three shots
“Heating up” damage no longer partially resets when turrets switch targets
MAX DAMAGE 225% over four shots ⇒ 220% over three shots
6.22:
Inhibitor and Nexus turrets melt enemies with lasers ⇒ fire laser bullets like all other turrets
5.12:
Lightning Rod no longer slows (previously 10%).
Lightning Rod no longer reduces the target's damage (previously 15%).
4.20:
New item Lighting Rod
Ignores 82.5% of the target's armor.
Unique Passive - Lightning Rod: Deals damage in a continuous beam and cannot be dodged. Deals up to 125% additional damage the longer it targets the same target.
With every hit, the beam gains 6 heat until 120 heat is reached. When the beam switches targets, heat is reset.
Gains +1.05% damage per point of heat, up to a maximum of +125% extra damage.
4.11: PENETRATING BULLETS: Turret damage gained per hit increased to 37.5% from 25%.
WARMING UP: Turrets now finish warming up after 2 hits from 3.
Turrets select targets slightly faster.
3.5:
Turrets now deal 105% damage to champions for the first attack, increased from 94.5%
Turrets now deal 25% additional damage for consecutive attacks on the same champion (max 225%), increased from 20% (max 210%).
Source: http://leagueoflegends.wikia.com/wiki/Turret
As you can see it's only been buffs except for the changes in 4.20 that got reverted in 6.22 - but those only affected inhibitor and nexus turrets, made lasthitting hard as hell and damagewise.. Well, people complained when Laser has been introduced and they complained when it got removed. But turret damage got buffed in season 3 and in season 6, so I really don't know what you are talking about.