In earlier expansions, such as BFA, your armor and block kept increasing during each season, with only expansions to bring it back down (similar to your secondary stats), this was the case in PvP at least.
In PVE, you had a "k value" armor / block penetration that was increased between each new raid to make your armor and block worse when fighting newer enemies.
Wowhead has a good article explaining the K value in depth:
Changes to Mythic+ Scaling on the 8.3 PTR - Base Season 4 Scaling, K Value, Dungeon Changes
This has changed in 9.1. I earlier thought (since at the start of the season our block and armor got reduced considerably) that Blizzard would manually reduce our defenses at the start of a new patch. But it seems like the change is to the system, so now it compares your ilvl to the enemy ilvl.
In PVE, this doesn't really seem to change anything, since "against current target" will make you stronger against the enemies as you obtain gear since their ilvl stays the same (think of it as if enemy NPCs had a hidden ilvl, which differ depending on difficulty and how old the content is)
However this system is now active in PvP. Our Ilvl is being compared to determine how strong your armor and block is when fighting the enemy.
This means that if you are undergeared, your defense will be worth way less than the enemy player, and their defenses against you will be even stronger!
The power difference if you are undergeared is even more severe than you might have thought!
But even worse, is that since we all gear ourselves, our ilvl goes up, which means we are facing "stronger" enemies compared to earlier seasons. This means that our armor and block value is lowered.
As we all gear, the armor and block is not able to keep up with the reduction from facing enemies of higher ilvl. Two people of 226 ilvl fighting eachother, seem to have higher armor and block damage reduction, compared to two 259 ilvl players fighting each other.
I do not mean that gearing is a bad thing that makes you weaker. But WE together by obtaining higher and higher ilvl gear are seeing our armor and block turn worse and worse.
For example, in the video when I turned on warmode and received higher armor, it still made me lose % damage reduction.
This is probably a big reason it feels like Warriors, WW monks and a lot of other melee do insane amount of damage, since armor used to increase between seasons to balance out physical damage done, but now it works the opposite, with physical damage turning stronger instead of weaker.
