As ZDoom development is an ongoing process and it is being constantly updated and changed, new features will sometimes be added that render older features obsolete. For this reason, it is often necessary to "deprecate" such a feature that is no longer needed.
ZDoom features are typically considered deprecated when a developer has implemented a newer, better and/or more generic method of recreating an existing feature. Many times the old feature is no longer useful and would be a burden on the developers to maintain alongside the newer features. This can be compounded by the fact that the older code may often have been a hack placed there by the original id Software/Raven developers to work in a very specific manner.
Once a developer has determined a feature to be deprecated, the wiki will typically be updated to reflect this information, and it is no longer recommended that authors continue to use the feature in future projects. In practice, the feature will usually be maintained by the developers so that old WADs that rely on them still work, but will no longer be updated and in rare cases may break when newer updates to the engine are implemented.
For the sake of information archival, it is not the practice of this wiki to remove information about a deprecated feature. Instead, the page will typically be updated with a warning and a link to the newer, preferred feature will be provided. Typically these wiki pages will not be updated beyond this point, unless the information is found to be inaccurate.
A note to wiki editors: Deprecation is something that should only be decided by ZDoom's main developers. If you see a feature which you believe is no longer needed, please do not assume that the feature is deprecated and add this tag to the wiki. Instead, please post the question on the forums and/or the article's discussion page inquiring as to the status of the potentially-outdated feature. Remember: When in doubt, ASK.