Introduction of the
msrv flag or Rust version
cfg would be breaking by itself, so it can't help existing Rust/Cargo versions, but only solve the problem in the future. This means that the best time to release it was many years ago, and now is the second-best time to release it.
It does need some adoption, but because Cargo compiles crates starting from the leaf dependencies, it only needs adoption in a few popular leaf crates. I estimate it can be literally as little as the top-10 worst-offending crates to make an impact on the entire ecosystem.