Extension Key Registration has moved
What was the cause of the issue?
The page "Extension Keys" is not available in the main navigation anymore. It is still visible in the subnavigation however. In this page the message "We are currently maintaining this area. Please have some patience." appears.
https://typo3.org/extensions/extension-keys/
The registration of extension keys has moved to the new TER imho, and is available behind a login https://extensions.typo3.org/my-extensions/
Solution
First step: Explain this on the old page and add a link to the new TER extension key manager.
Second step: Remove the page, add a redirect to https://extensions.typo3.org/my-extensions/