It was displaced around te same time as the 'new' high speed exit was built on 04. What is now QR. Prior to this on easterly ops runway capacity was affected by the distance of the high speed exit from the touchdown zone. Medium traffic generally took a lot longer to vacate than they did on westerly ops. To the point where it was common to have an extra mile between landers to get a departure away.
However BAA displaced it and built a new rapid exit further on from the current one. Then pilots of a certain Irish carrier would always plan for the second exit(closer to their stands) and thus the whole thing was a bit of a waste of time as the distances were the same. (300m displaced threshold- new exit 300m further on...)
Displacing threshold also led to some very restrictive rules on the northside of the airfield to protect the glidepath.