I have a Z445 48" that I've mowed 5 acres and a few others places for the last 14 years. Is it possible that the metal on the deck is worn thin? It's mowing uneven and it appears the deck is bent. Spindles are fine. Has anyone straightened out a deck?

How is the cut quality?
Over time the lift (raise and lower) linkages wear so the deck begins to sag, I would try leveling the deck first.
The deck shell shouldn't be bent unless you've hit a stump or large/hard material.
https://www.sunequip.com/news/dealer-news/how-to-level-john-deere-zeroturn-mower-decks-in-6-steps/

The middle blade is scalping, the outers look about right. The cutting edges of the blades don't line up

Yeah, I caught a chunk of 4x4 between the blade and deck one time that bent the deck housing on a 48” JD mower.
I had to pull the deck off , remove the bearing housing, shim on the bottom of the deck and use c clamps to straighten/ level the deck where the bearing housing mounts.

Raise the mower as high as the adjustments will allow and look across the blades from the exit chute and see if the center one is bent down lower than the others.
It's fairly common and easy to hit a rock, block or chunk of limb and bend one or both ends of the thin consumer grade blades and have one of a set cut higher or lower than the rest.

The consumer grade ones are usually 1/8" thick with a rib stamped down the center extending from center to where cutting edge starts.
The HD commercial ones are usually 3/16" or thicker and straight with no rib stamped in them.

The HD version is usually made from better metal and holds edge longer but if you hit rocks, all bets are off!
The HD version can also be sharpened more times as there is more metal.
Using a simple stepped cone lawnmower blade balancer after you sharpen it will help stop vibration so the deck doesn't shake itself apart and crack.

These were HD blades, I initially thought they may have been bent so I put new blades on it. The deck is bent and the middle blade is sitting about 5/8 lower than the outside blades. I thought the deck was wore thin in that area but a hammer doesn't bend it back very easily. I worked on it a couple of hours and said the hell with it and bought a Kubota.