Fifteen years of iterative design means that the World of Warcraft that existed in 2006 is dissimilar to World of Warcraft: Battle for Azeroth as it exists in 2019, even if you’re playing the same content. The cumulative effect of thousands of tweaks means that pinpointing exactly what made Classic so special is a nebulous undertaking. Flying mounts, for instance, were a welcome addition when they were introduced in The Burning Crusade. It eventually became clear to players, however, that the convenience and fantasy they offered came along with a number of significant drawbacks like reduced interaction with topography, enemy NPCs, and other players. In spite of this, flying limitations are met generally with negative feedback on the part of BFA’s community. Such conveniences are difficult to unlearn.
Making "new" guides seems kinda out of place. Not only is there still alot of information from back then available, there came alot of information up for minmaxing available for both leveling and raiding, ontop of various bis gear lists that cover patch progression aswell if necessary. Wether people like or dislike pservers does not make the various information found on their forums less valuable.
That's not to say that everyone has an unrealistic vision of what vanilla WoW was like. There were already plenty of people on the WoW Classic forums pointing out that perceived bugs are just recreations of the original game. And as noted before, a flaw to one player is a key component of the original positive experience to another. WoW Classic will surely please plenty of purists in spades. But this goes to show that it won't be for everyone who has fond memories of the game circa 2006.