Looks like the short 21 has become the shorter 21.
When the diversions started in Chester le Street on Monday I had expected the short 21 to follow the diversion route south bound to its normal terminus at the south end of Front Street then run as normal north bound. However the short 21's are terminating at the Market Place then heading out north past the Market Tavern.
GNE have not made this change clear on their website.
RE: Go North East: Upcoming Service Changes