A lot of the things ‘credited’ to players were done with help, many of the most powerful threats were canonically dealt with not only by a full team of strong champions, but by NPCs as well that are quite powerful, far more so than the players, Order Hall leaders or not. It’s also important to remember that just because a threat was in a later expansion, that doesn’t necessarily mean it’s stronger lorewise than a boss in a prior expansion.
All of the threats in Vanilla WoW she could have taken down, yes. The strongest thing we fought in Vanilla, lorewise, was probably the mostly dead C’thun, which Aegwynn at the height of her powers could safely nuke. The mental image of Kel’thuzzad trying to fight her is actually pretty hilarious.
Burning Crusade, again, she’d be able to handle herself. Both Kil’jaeden and Illidan were defeated with outside help/interventions, and Illidan, while powerful, wouldn’t be able to contend with her, neither would any of the other bosses in the prior tiers.
Wrath is a bit trickier. Obviously the first tier would be doable, Malygos is the most powerful boss of that tier and he was defeated with help/intervention from the Red Dragonflight. Yogg was beaten with the help of the Keepers (all of them giving their blessings is probably the canon version, otherwise you wouldn’t need to free them at all). Arthas is an interesting beast, because the champions didn’t actually beat him either, they stood no chance and it was literally divine intervention. At worst, it would play out the same way.
I don’t see any issues with the Cataclysm era bosses being defeatable by her, only Deathwing is the real massive threat, and we had help taking him down as well. Arguably that’s the fight where the champions did the least to actually save the day.
All of MoP wouldn’t be an issue. Thunder King would be the toughest fight but she’d be able to take him down.
WoD is interesting because the standout fight is… Archimonde, at the end. It would be a tough battle, but given what we know of the Guardians, as well as Aegwynn’s experience and power at her prime, she’d win.
Getting to Legion, all of the final battles/boss fights we have help, and thus so would she. Gul’dan we have the Eye and Khadgar’s help, plus Aegwynn would be able to beat Gul’dan normally anyway. Avatar of Sargeras is a husk and not nearly at full power, beatable. Kil’jaeden is a tough fight, but isn’t fought alone either. And Argus is fought with the help of literal gods.
BfA’s threats also all have outside help and interventions, though we can probably discount Siege of Dazar’alor, as I doubt she’d participate. And if she did, it would be a cakewalk for her. Azshara would be an interesting fight, but with Aegwynn, Lore’themar, Thalyssra, and Jaina all together? It would be a win. 8.3 raids would probably play out the same, though Aegwynn would probably gank Azshara after freeing her.
So yeah, things would play out the same, she could do it all. Medivh basically nuked a massive army of trolls to the point of breaking the power of their tribes… by accident. Aegwynn with centuries of experience is beyond even most lore characters we know now.