The tv-show-missing-links variable defines if missing links within the scope of the variable are rendered as links or not.

Globally, the variable is set within the PageTemplate and can be changed in the $:/ControlPanel under Settings - Show missing links to tiddlers

If tv-show-missing-links is set to no, missing links are rendered as plain text

If tv-show-missing-links is set to yes, missing links are rendered as wiki-links