The purpose of marking a page as deprecated is to inform visitors of content that is no longer valid and should not be used any further.
There's 2 related templates, Template:Dated, for pages with stale or otherwise out-of-date information–but that in general just needs an update, and Template:Graduate, for pages that should be graduated to https://github.com/sugarlabs/www-sugarlabs and then marked as deprecated.
Note: Before marking a page as deprecated, make sure you have confirmed that the page content really is no longer valid (or that it has been replaced by more current information). It may be helpful to talk with someone who has recently edited the page, in case you have doubts.
To deprecate a page, include the Deprecated template:
{{Deprecated | some text pointing to an [[alternative page]] or http://some.url or just explaining why.}}
The template takes an extra (optional) parameter that can basically be any text and will be displayed right after the warning sign.
Deprecated Category
All deprecated pages are listed in Category:Deprecated pages
An example of a deprecated page is Wiki_Team/Roadmap