This article also explains TFA's confusion about not finding examples of 'lede' earlier.

The variant spelling was an old practice, well-known to Safire & the other "old hands of journalism" mentioned (such as Herb Caen), as inside-jargon – but its initial practical value came from its distinction from the 'common spellings' for similar concepts.

But Safire writes about it, in 1990, because just around then it's starting to sneak out into wider usage. Safire concludes his reporting and wordplay-around-the-word with:

But [lede] has earned its place as a variant spelling, soon to overtake the original spelling for the beginning of a news article. That's lexicographic news, but I have learned never to put the story in the lede.