Ultimately, though, it is pretty much impossible for a state to outlaw 3rd trimester abortions.
The opinion’s plain language permits later term regulation, so I don’t know to whom you refer when you say it was interpreted as de facto prohibition on abortion. But you’re 100% correct that Casey had a major impact, as did other cases:
Webster v. Reproductive Health Services
In this case, the Court upheld several abortion restrictions, and modified the Roe trimester framework
Gonzales v. Carhart
On April 18, 2007, the Supreme Court handed down a 5 to 4 decision upholding the constitutionality of the Partial-Birth Abortion Ban Act.
The U.S. Supreme Court has ruled that the term “partial-birth abortion” in the act pertains to a procedure that is sometimes called “intact dilation and extraction” by the medical community.[2] Under this law, “Any physician who, in or affecting interstate or foreign commerce, knowingly performs a partial-birth abortion and thereby kills a human fetus shall be fined under this title or imprisoned not more than 2 years, or both.” The law was enacted in 2003, and in 2007 its constitutionality was upheld by the U.S. Supreme Court, in the case of Gonzales v. Carhart.
The bottom line is, this statement, “Roe v. Wade stands for the proposition that abortion CAN’T be regulated” is entirely false. It can, as expressed in Roe (see my excerpted passages), and has been, see cases above.