lifetimebound was already proposed by
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0936r0.pdf

There was consensus in favor of addressing the problem in C++20, but
no consensus in favor of an attribute. Then, the paper was abandoned
by the authors.

If you want to work on this, you should get in touch with Richard and
Nicolai (CC).