It started in season 1, actually, when RIOT decided to design around Ashe as the pinnacle of balance for whatever reason.
Bruisers were mostly created in late season 1 through most of season 2, with a smattering of 'em since. The ones in this particular range though, all were designed with the capacity to kill an Ashe while she had Flash available in mind. This then cemented flash's place in the meta, and slowly made it more and more difficult to touch, but it also established the president of the melee vs ranged match up for LoL.
Fighters were supposed to have windowed opportunities where they could either A, get to a target and CC them, while bursting them down, B, get to the target then keep dashing/chasing after them, C stick to targets like glue, or destroy them utterly, but have trouble reaching them in the first place.
Well, that was all based around the target being ranged, with spamable soft CC and also hard CC for peel, and being able to create at least a 450 unit gap (though I think flash was still 800 range when this first started) at the press of a button. Against an ADC like Kog, with much less peel, those fighter's kits no longer felt fair.
And so ranged AD players bitched and whined to RIOT and then chain nerfed most bruisers into shadows of their former selves (better nerf Irelia, and Olaf in general became memes for a reason) while gutting their itemization, destroying almost ALL of their base stats in some form or another, as well as removing parts of skills, lowering ratios, increasing cooldowns, etc.
Meanwhile, RIOT has also steadily been nerfing all defense itemization wholesale. Compare current Randuin's to Seasons 1 + 2, or how about Warmogs giving less than 65% the hp it used to, let alone Thornmail (rip), or Sunfire Cape, or Frozen Heart. Not to mention, FoN, HoG, Shureliya's, Soul Shroud, Runic Bulwark, hell, even fucking Leviathan were all removed.
While also buffing Crit, nerfing the cost of AD and making it way more accessible, increasing the availability of Attack Speed, and then throwing way more defensive stats and abilities on the ADC items, and also giving them powerful unique passives on items that are themselves even cheaper. Oh, and they buffed the ever loving shit out of support items, so now all supports can heal and shield instead of just enchanters, and they can peel bruisers off their intended targets much easier.
So defensive is now way weaker, Bruisers themselves were directly nerfed in almost all cases, ADCs are stronger, their supports are way stronger and have more tools to peel people off with, and they haven't tried to do the sweeping, systemic changes they need to to help out those who have been left behind.
They've recently been trying to help tanks after fucking them over something fierce, and its only after that put them in a good spot that they've been faring better than bruisers-- outside of niches in competitive play affecting less than .001% of players.