The situation:
Running heroku update says you are already on the latest version while every command still warns that a newer version is available. One common cause is a stale local version cache: the CLI checks a cached latest-version file, and that cache can keep announcing a release that was rolled back or that your install channel never received. A Heroku team member confirmed the fix is to force a refresh of that cache, and the reporter later confirmed the warning stopped appearing. When the warning comes from an install channel that no longer publishes updates (snap, for example), the real fix is reinstalling through a supported channel like apt.
What actually fixes it (verified in the thread):
Force a refresh of the CLI's version cache: run HEROKU_FORCE_VERSION_CACHE_UPDATE=true heroku apps (or any heroku command with that env var set) and the phantom update warning should disappear. This was confirmed by a Heroku team member and by the reporter on the issue. If the warning returns and you installed via snap, note that Heroku no longer supports snap installs: uninstall the snap and reinstall via apt following the official docs instead. Source: https://github.com/heroku/cli/issues/995