Can't be specific I'm afraid (I wasn't alive)- there's a chance it was much sooner than that, it could have only been used a dozen-or-so times in the very early days of the franchise. There are literally no examples of its usage that I'm aware of in the context of a junction, though one of the pres tapes that went around in the 90s had an example of a couple of minutes of it 'clean'.
Either 6th September 1999 or 30th August 1999 if I remember correctly. They changed again in about a year later and again when ITV became ITV1.
Those promos were originally used from the 1998 relaunch, as that was when that ITV logo was introduced. That example has the hearts logo on it, so it dates from after November 1999, as those promo styles were being used right up until October 2002, having been modified for the new corporate hearts as of 1999.
I do recall someone somewhere saying that the reason the ITV logo is so huge on that end board (and blurred in with the image) is that on the first iteration of the hearts NPU promos the more vocal regions had a habit of blanking out the ITV logo with their own, and they couldn't do that anymore with the new design!
The third version of the end board (introduced Autumn 2000) had smoky hearts, and the final one from Autumn 2001 was a coloured wavy line.