I'm using Windows 10. I uninstalled all my old versions of 68.0 and installed the released version of Thunderbird 68.0 from thunderbird.net. It does not have a link to the release notes in the help menu in the menu bar or in the help menu you reach using the hamburger button. I expected a menu item that would display the contents of https://www.thunderbird.net/en-US/thunderbird/68.0/releasenotes/
Normally its Help Contents, Whats New, Release Notes, Troubleshooting Information, Restart with add-ons disabled and About Mozilla Thunderbird.
Instead it was: Thunderbird Help, Thunderbird Tour, Keyboard Shortcuts, Get Involved, Make a Donation,
Troubleshooting Information, Restart with add-ons disabled and About Thunderbird. I tried each to verify the link to the release notes wasn't just mislabeled.
I also used Go -> Mail Start page to see if a link to the release notes was moved to the welcome page for some reason. No sign of it.
I searched for releasenotes using the config editor. I found app.releaseNotesURL with a value of https://live.thunderbird.net/%APP%/releasenotes?locale=%LOCALE%&version=%VERSION%&channel=%CHANNEL%&os=%OS%&buildid=%APPBUILDID%