Asking questions is always a good thing so don't worry about it. It makes us think and check our own understanding too, so we also learn something :-)
A nice succinct post by Pavel Khaipov; Copy-on-Write in Modern C++: Why the Standard Dropped It - and Why Qt Still Uses It - https://www.linkedin.com/posts/khaipov_copy-on-write-in-mode...
A detailed post by Andrii Nikishaiev; Why Copy-On-Write (COW) removed in C++? - https://www.linkedin.com/pulse/why-copy-on-write-cow-removed...
The std document on Concurrency Modifications to Basic String - https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n25...
Finally, C++11 tightened the specification for std::string viz. contiguous memory, some methods must have O(1) access time, when references/iterators/pointers can/cannot be invalidated etc. which doomed COW.