They stay with the code as long as the code is absolutely untouched, which often is a lot shorter than a comment about code remains relevant. Code gets moved, reformatted, ... without details about it becoming stale.
Unless you have large chunks of code moved around wholesale, you can usually track all commits that touched a particular line of code. I've done that while debugging codebases that were over a decade old. And seeing those commits was incredibly helpful, despite the code itself being heavily commented.