I will be honest in that I don't understand the amount of vitriol that I'm reading in this thread. If we look at the path the Illinois has taken so far:
Start: Illinois is a 203mm gun Battleship that is interesting. The concept has been requested for years and get slated on our internal roadmap for creation and implementation.
Initial Concept: The initial hull is a Battleship hull that has weaknesses. In direct confrontation in might not withstand overmatch that exists from some battleships she could see. Also, the DPM is intriguing but the Battleship dispersion is a bit of a limitation. As such, she's given improved secondary capability as a value add. It's not uncommon to see secondaries married with weaker caliber main turrets (see: Pommern at the same tier with improved accuracy at the expense of shell size). Also, Slot 3 could use the Secondary Focused Upgrade as an option instead of the +range Slot 3 (because shell float could be seen as a common limitation).
Testing Begins: The 203's are US so the shells are floaty. Also, the dispersion is notably worse than Cruiser-level because she's a Battleship with Battleship Dispersion, albeit with access to the US Slot 6 -dispersion Upgrade which is likened to make Battleship Dispersion into Battlecruiser Dispersion.
Data is pulled from players playing the ship.
Revision A: The 203's are buffed in reload time and given better Dispersion by default. The Slot 6 still exists and could further effect the 12 gun salute into better-then-battlecruiser dispersion? This gives a natural increase in power at further ranges as you could expect more consistent damage (if you can deal with floaty shells). Gun-range combined with non-Brawling BB hull leads to less likelihood of brawling then existed before, which the DevBlog mentioned was of negligible merit anyway. Since the Secondary power wasn't showing relevant, the power is transferred to the main guns (damage comes from Main Guns and Secondary Guns, so take away from one and give to the other). Lastly, HP reduction seems sensible as Montana-style US heals are very powerful and increased expected engagement range likely translates to additional safety/damage resistance.
Cue next round of testing.
Illinois is a Battleship with 203's. That's what people have talked about since her announcement, and the recent changes move her further in that direction. It's a very understandable direction.
The thing that stands out the most is the size of the health reduction, which really does seem impactful. When looking at Buffs and Nerfs during the Testing process, Buffs indicate overperformance and Nerfs indicate underperformance. If it feels like the Illinois was nerfed overall, that would likely indicate that she was overperforming in terms of test data.