I believe this is https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p14... and seen in 4.1.2p6 of https://eel.is/c++draft/intro.abstract, and then p7 lists what the time travel gates (observable behavior) is.

Now if this actually made it into the C++26 standard, I've not paid to get the official one, but it's in the N5050 draft: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/n50...