My point was that in ARR and even HW, you didn't need to delve deep into any systems to be able to clear content whether that was on as casual or more hardcore level. It wasn't required of you then, and its not now. And you even agree with my point of that!
Right, but people still felt pressured to get things such as Provoke, Swiftcast, Protect, damage buffs and so on. Extending upon such systems may have got them to a point where you actually needed to do it, and extending them seems to be what you're implying with "the systems weren't that deep, but they could have been".

I have often thought that maybe content could have been done without Provoke then, such as having the PLD off-tank, then use Cover, have the tank invuln next one, Provoke next one, invuln next one, let aggro lapse back, then Cover, then Provoke and so on.

But realistically players felt that it was and treated it as such. In casual content, Provoke was expected if the tank died. Swiftcast wasn't needed but, again, players felt that it was because otherwise they are casting rez for 7-8 seconds. And Protect - how dare a healer not cast protect at the start of a duty? Everyone did this!

what food should they be buying?
Since SE ordered it by item level now, you just look at the highest one up that has crit, dh or det in any combination and it is probably a good choice.

what materia should they be melding?
Usually not going wrong with crit, dh or det either. It's mostly irrelevant as well. Melding one of these is a lot better than melding nothing.

You weren't expected to be min-maxing to a degree that needed you to be casting a GCD with 90% or more uptime on nearly all party members to get clears on harder content, an aspect you do need for raiding today.
I would say this was expected in Heavensward, although at the time, for melee jobs, it interrupted your combo to attack from range.

And although healers may not have all been doing DPS in ARR, that was when people were noticing that healers could be doing more.

In Casual content then, it wasn't expected of you to know everything
This is debatable. When I was new to the game and tanking, I was attacked a lot in dungeons. People regularly exited the dungeon near the start, they attacked all the little nuances about tanking that I wasn't doing (when those nuances existed), or when they incorrectly thought I wasn't doing them. They would run ahead and pull. They would attack me for not knowing a certain pull strategy for a specific dungeon. They would call out people spamming an enmity combo when it wasn't needed and things like that.

But there were plenty of people who were nice as well and it depended which roulette you were doing (leveling was really pleasant, the level cap roulettes were not).

Despite how much people desired dungeon players to "know everything", the fact is they never have, and never will, because a lot of the people you meet in them are new, returning or casual players. But has it been expected? Sure.

when it comes to dipping your toes into Savage/Extreme content today you are required to know your stat priorities with Materia and food. Buy the pot of your main stat. And you had better know your rotation near perfectly with over 90% uptime at all times. And whatever you do never miss the 2-minute meta burst for your class or else you'll never clear even the first boss.
Well all this depends. You've always needed all that to an extent when the content is new, but when the content gets older and you can get gear from alliance raids, hunt upgrades and so on, you've always had more leeway as well.

I think recently, the 2-minute burst window and removal of other advantages such as piercing and slashing damage have also meant SE doesn't have to scale DPS checks with the assumption that all these things aren't being done, because they removed the damage types and forced buff alignment, but the fights still get more leeway with gear regardless.

know the mechanics WELL before you actually step foot in the raid since so many of them are body checks either going into, during, or coming out of the mechanic. I'd say the game is actually less casual friendly now because you have to do your homework before you even understand what happening around you in current difficult content.
People have always watched guides and "done their homework" for raids. In fact, a huge percentage of people used to watch MTQ's guides for mere dungeon runs (because dungeons were less intuitive then and you can see this in the Hard Mode dungeons still). Going even back to ARR, MTQ and MrHappy made extreme and coil guides.

Personally, for Heavensward, I used the savage guides produced by some raid FCs on the internet that they pushed out week 1. Their strats weren't always the same as what PUG strats ended up being but they were solid.