Take a piece of paper and just loosely bring the top edge down to meet the bottom edge. Let it go, and watch it spring back. That’s elastic deformation. Atoms and molecules stretched, compressed, or twisted in response to stress, but did so reversibly (no bonds were broken, no atoms suddenly had new neighbors). Now fold that piece of paper again, but this time do it hard enough to crease it. That’s plastic (permanent) deformation. The material in the region of the crease will never have the same properties again. Even if you unfold it and stack heavy books on it to flatten it out, you’ve permanently altered the material (and probably not in a desirable way, or it would have been purposefully done in the manufacturing process.
Plastic (permanent) deformation makes changes to metals that you can’t come back from, short of recrystalizing the material. Typically there’s hardening (often accompanied by embrittlement). Sometimes metals are purposefully plastically deformed to “work harden” them (like the material of a sharp blade — the process of forging is basically a purposeful plastic deformation process).
But the (space)frame of a vehicle isn’t meant to be plastically deformed. Your car’s frame will plastically deform in a severe impact, but that’s to sacrifice the car to save the occupants. You can’t just unbend the crumpled frame and put it back in place. It will never be the same again — its desirable properties are forever changed (again, short if our being scrapped and reconstituted through some recrystallization process).
That crash into the planet deformed parts of that ship beyond belief. Coupled with the material fatigue from normal use, I can’t imagine it ever being space worthy again (short some magical technobabble tech that could basically reconstitute something of that mass at an atomic level, more efficiently than simply rebuilding something new).
I think in one of the Shatnerverse novels, it was mentioned that after salvaging whatever surviving stuff they could from the saucer, it was blown to smitherines (to prevent the nascent civilization on the nearby inhabited planet from ever finding a trace of it and being culturely contaminated. That made sense to me, and that’s how it went down in my head canon.