Update: He corrected himself when I was writting my post. Ignore this post.

No, the closing tag is optional (in HTML 4.01). Note the "O" for "Optional":

<!ELEMENT LINK - O EMPTY               -- a media-independent link -->
<!ATTLIST LINK
  %attrs;                              -- %coreattrs, %i18n, %events -
+-
  charset     %Charset;      #IMPLIED  -- char encoding of linked reso
+urce --
  href        %URI;          #IMPLIED  -- URI for linked resource --
  hreflang    %LanguageCode; #IMPLIED  -- language code --
  type        %ContentType;  #IMPLIED  -- advisory content type --
  rel         %LinkTypes;    #IMPLIED  -- forward link types --
  rev         %LinkTypes;    #IMPLIED  -- reverse link types --
  media       %MediaDesc;    #IMPLIED  -- for rendering on these media
+ --
  >
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