I have no idea why that isn't standard practice in every codebase. I should be able to figure out your code without having to ask, or dig through issues or commit messages. Just tell me what it's for!
If someone didn't take the time to update the comments and the reviewers didn't point it out, then you've probably found the bug because someone was cowboying some shitty code.
Outdated comments are often way worse than no comments, because they can give you wrong ideas that aren't true anymore, and send you off in the wrong direction before you finally figure out the comment was wrong.
if (X) assert(false); // we never do X, ever, anywhere.
if (exact same X) { do_useful_stuff(); }
enum kinds { writers; readers; updaters; }