https://en.wikipedia.org/wiki/Bill_of_attainder
The constitution commands the congress to do this "No Bill of Attainder or ex post facto Law shall be passed."
The people who wrote the constitution knew how the crown was abusing people. They wrote it specifically to stop that. It is also attached in many cases to state constitutions.
Specifically in this case
https://leginfo.legislature.ca.gov/faces/codes_displaySectio...
Also I am not a lawyer so ....
Perhaps a middle ground can be reached. Where there is some sort of incentive for the inheritors of who did it, to fix it?
If the settlement was reached based on incomplete or false information or representations from the company, the government may still have recourse.
So even if the settlement per-se remains in force, a substantial fine or even a new criminal case may still be possible.
It has more to do with tolerance of callousness and stupidity in previous decades than it has to do with there not being a specific law against dumping specifically DDT into the ocean.
There are even plenty of examples in the page you link of justifiable ex post facto laws. For example war crimes.
To be honest, because of the devastating long term effects of environmental pollution, and the deaths it has caused, I would quite happily argue that laws applying to it could be ex post facto. Crimes against future generations.
I remember covering this in my philosophy degree and you can argue about it for hours. The example I remember was Nazi collaborators who reported on Jews or resistance fighters or even falsely accused neighbours.
They got people killed, often maliciously, but what they did was perfectly legal at the time. Should they have gotten away with their complicity or should they have ex post facto laws brought against them?
In the time of the US founders, the scale of damage a single human or company could conceivably do for profit was a lot less.
However, there was always a principle of civil liability of damages caused, so this should still be applicable.
The idea that a government can make something illegal and retroactively punish previous offenders is so colossally bad that in the United States the Founding Fathers wrote a prohibition against it into the Constitution.
It's not an amendment, such as the right to free speech. They wrote it into Article 1.
https://en.wikipedia.org/wiki/Ex_post_facto_law.