Only Battle for Mt. Hyjal was available from the start of T6 content since Black Temple came with patch 2.1, however no guild in the world managed to beat pre-nerf Kael'Thas in Tempest Keep before patch 2.1 rolled out
Abit of the problem with bosses in T4/T5 in the start that it had a rather huge difficulty gap. Some bosses like High King Maulgar in Gruuls Lair, Void Reaver in TK and most bosses before Lady Vashj in SSC were not that hard to deal with after giving people a few tries to see the encounters, but then you had Hydross whom was very challenging as an opening boss in SSC (mainly due to the bugged resistance in the start, randomly 1shotting fully resistgeared tanks (in correct resistance gear ofc) + very unforgiving mechanic with the phase switches from frost to nature and back).
Al'ar in TK where you either had to sacrifice a team member every meteor (most guilds abused full-raid soulstones back then before blizzard changed it especially for this encounter) or have whole raid stack to split the damage as much as possible and also guilds strugged with killing adds while dps'ing Al'ar at the same time so they just offtanked them and nuked Al'ar before tanks got overrun with adds.
Magtheridon was another boss that had an unforgiving mechanic with the cube-clicking that required top teamplay and coordination, and High Astromancer Solarian with the Arcane debuff mechanic before introducing the bomb mechanic later that forced some players in the raid to wear full arcane-resist gear and stay behind raid to soak and have the debuff jump between them.
Now, TBC is 8-9 years old so with the amount of knowledge, strategies and theorycrafting over the years should make everything easier to deal with, but what I'm more interested in regarding the PvE progression is the change of consumables that was introduced in patch 2.1 with battle and guardian elixirs if we will have that ongoing from the very start or if Nost will be able to change it once they roll 2.1 content (2.1 also buffed tons of existing lv.70 epics as they were undervalued, hence why most top guilds back then was still running in decent amount of T3) versus giving us pre-nerf bosses. There was no forced gating back then since players were new to the content aside from some bosses that got tested on TBC Beta.
Personally having battle/guardian elixirs, pre-buffed Lv.70 epics and pre-nerf bosses in T4/T5 would make raids very interesting despite the content being 8-9 years old.
So I'm also very interested in how Nost will handle the PvE progression roll-out, both mechanic wise and gating wise in terms of attunements or forced gating to "slow down progression".