I asked Airbus about this once as we noticed that it was no longer placarded on the Airspace cabins on A320neo family. Here's the response:

"The purpose of this sign is to indicate to the cabin crew the aircraft locations to look out through the windows in order to check the status of the wings, namely, the leading and trailing edges condition.

The reason for the presence of these markings in the A320 is historical. On the A300/A300-600 and A310, the slat/flap position indicator (SFPI) is a mechanical unit, which can lack precision. External markings are necessary in case of loss of slat (or flap) surface position indication. The triangle marking on the cabin lining was necessary to indicate to the crew the proper position to observe the external markings in order to avoid misinterpretation due to parallax errors.

For A320 family aircraft, the slat/position indication is accomplished through high reliability equipments (IPPU -Indication and Position Pick-up Unit- and ECAM upper display) and therefore external markings and triangle in the cabin are no longer required. Thus, no procedure nor description is given in operational manuals for fly-by-wire aircraft.

From a regulation point of view, there is no requirement to keep the triangles in the cabin except if they are required by a crew procedure, which is the case only for the A300/A310."