Since the original release in 2004 the ways in which your damage output was impacted by additional reductions with their own caps and calculations has been reduced a lot with Parrying at this point being the last remaining one (afaik).
Hit and Dodge cap got removed.
There are no more glancing blows or partially blocked attacks.
For casters specifically spell resistance is no longer a thing.
But why do melees still have to deal with bosses turning around towards the melee camp or ranges behind them for specific targeted spells or cone attacks? Right in those moments you still have that 5(?) % chance of getting your auto-attacks or abilities parried leading to massive losses on debuff uptime or ressource generation (greetings from an Arms Warrior). Same goes for pulls that don’t really allow for charging/attacking from behind or bosses turning for a split second because someone else overaggroed or got the first hit before the tank. Especially as an Arms Warrior it can completely ruin your opener if your first melee attack is getting parried.
Outside of specific spells/CDs parried attacks are no longer a thing in PvP or 5man content (due to NPC levels being only +2 and not +3). But why do they still have to be accepted when fighting raid encounters?
I know there used to be the original reasoning of teaching players to stand behind an encounter (or rather punch dragons in the kindeys) and not eat cleave. And getting parried is something you don’t want to see (regardless of when Parry Haste got removed) so you stand behind a boss.
Today the average player is aware of these mechanics and can adapt to cleave mechanics and doesn’t need the DPS carrot on a stick to move accordingly. On the other hand many encounters require a much more agile and dynamic playstyle and movement. Depending on their hitbox you also can’t trail behind too much during movement/kite phases. Add to that: constantly reorienting yourself because of adds and other effects (Anduin shadow phase for example) And as mentioned above: Outside of the anticipated turns you still have to deal with that random turn around + 5% of losing out on damage output.
Edit:
Tanks are automatically “parry capped” because they stand in front of the boss (regardless of holding aggro or being the off tank). The days of swapping aggro and tanks on position on certain encounters are a decade old and encounters are much more dynamic these days. Expertise also got removed as a “must have” cap/stat.
If it was made sure that tanks never had to deal with parries because they couldn’t do anything besides attack from the front then you’d also have to make sure that melees have a guarantee that they can always attack from behind the boss. That is not a given thing these days, therefore: Parrying should be no more.
Also: There used to be modifiers like a “wide back” for Ragnaros in Firelands or a boss like Archimonde (HFC) not parrying (and back then) blocking at all because his defensive stats did not exist in that regard to allow for a better Nether phase. If something like that was justifiable years ago: Parry can go the way of any other defensive NPC stat by now.
In short: Is there any reason to still justify this solely for melee DPS (some suffering more than others) while other specs only think about their maximum attack range and nothing else?