Any boss that people managed to kill in retail would be relatively easy for high-end private server guilds of today. Many players have more experience than the total number of hours WoW had been up at the beginning of TBC, we have perfect foreknowledge about optimal tactics and raid compositions, more advanced theorycrafting knowledge, 2.4.3 classes are on average slightly more powerful than 2.0.3 ones and some guilds might even be more hardcore than old world first guilds in terms of schedule or how the guild is run (such as being able to completely disregard offering raiders a carrot during progression and loot council making mathematically best decisions instead). High-end private server guilds hold every single advantage.
That being said, well-implemented pre-nerf TBC T4/T5 content wouldn't be a pushover as opposed to, say, how vanilla raids get cleared on private servers (within an hour or two after anyone sets their foot inside). But if there were guilds actually serious about the whole thing, you're not going to see guilds "spending weeks on bosses like Al'ar". There's still room to go a step further while remaining "Blizzlike" and implement unkilled (such as Morogrim) or exploited (allegedly Vashj) versions of encounters - there's a lot of inference (but little concrete evidence) to be made about some of the content having been substantially more difficult before any documented changes, but at that point you're largely guessing for values and even mechanics (Morogrim apparently had Waterwalking Elemental adds in addition to murlocs, but show me a video of how they work) because any pre-2.1 video footage or logs is rare, never mind anything of high quality.
With such a surprise announcement, I'd be quite impressed if the server had consistently implemented 2.0.3 mechanics (Mind control on Vashj, Al'ar with numerous changes like phoenixes dying not causing him to lose health, rape patrols on Old Hillsbrad, double Sentinel pull in Arcatratz, Tempest-Forge Peacekeepers in Botanica doing arcane melee damage...), never mind consistently plausible pre-2.1 values. I don't know if even Outland has "correct" values on anything (even though, in principle, the most nerfed 2.4.3 values would be accessible by recording them from retail if you figure out all the modifiers they've gone through since, starting with -30% everything from 3.0.3). Spell data can be gained from WDB and post-nerf health is datamined, but melee damage on mobs would take quite intensive research.