Technical notes Redirection for Confluence
Never delete macro at top of the page

A macro appears at the top of all pages that have added a redirect.
Never delete this macro
The app must insert this macro as Confluence does not allow for a more traditional 301 Moved Permanently.
Getting to edit model when deny disabled
There is no direct way to get to edit mode on the page with a redirect if you remove the button to deny the redirect. This is also the case when the entire popup is disabled.
Workaround
Be quick. It must be done before the redirect page macro loads. Click anywhere on the page outside of the redirect popup (if has one). Then click the esc button on your keyboard. No harm clicking it multiple times.
If it doesn’t work reload the page and try again.
Legacy editor not supported
Redirection does not support the legacy editor.
Converts legacy editor page to new editor if save a redirect
Adding a redirect to a page still using the legacy editor will update the page to the new editor.
Moving to the new editor could impact your page formatting, often related to third party macros. For more details read this from Atlassian: Convert pages to the cloud editor.
Requirement to confirm before going to external URL
Atlassian requires the popup with a warning message and a confirmation button in order to go to external URLs. A redirect to an external site cannot happen without this popup. Atlassian does for security reasons.
In the future Atlassian might remove it or offer a way to white label URLs. Many users and developers have been complaining that this is too restrictive and Atlassian is open to finding a better way to balance security and ease of use.
HTML
HTML is not allowed in the custom message fields. The WYSIWYG editor is the only option. It allows to create headings, bold, italics, underline, monospace, bullet points or numbering.