First of all: Keep calm!
In several fields, deprecation is the discouragement of use of some feature, design or practice; typically because it has been superseded or is no longer considered safe – but without completely removing it or prohibiting its use.
wikipedia
For TiddlyWiki it means, that you should not use this mechanism for new content anymore! AND you should update your existing content!
Deprecated features have a marker. see: Custom styles by tag
Tiddlers tagged $:/deprecated