I think simplification coast means that the coast was once jagged (rough) thousand years ago, but due to the xxx Current, the coast's roughness has been eliminated and smoothened.

It depends on whether they are sandy beaches or muddy flats. Some examples: "Look in your atlas and find two large U.S. cities that are on flat coastlines and two that are on mountainous coastlines." missiongeography.org/water4student.pdf "The cliffs give way to a flat coastline with beaches and estuarine shores in Picardy, round the mouth of the River Somme." about-france.com/tourism/french-seaside-coast.htm "Sedimentation on the open-coast tidal flats of south-western Korea is controlled by seasonal variation in the intensity of onshore-directed winds and waves. As a result, an environmental oscillation takes place between tide-dominated conditions in summer and wave-dominated conditions in winter." http://www3.interscience.wiley.com/journal/118648482/abstrac...

currents have acted to erode the coastline to a less rugged, more even terrain

or coast line with interface ?
depends on coast
1.
" 340 km of southern Baltic coast ........It exhibits all characteristic of a simplification coast with its long sandy bars and lagoons and its steep slopes of glacial tills. It is therefore the result of a long and highly dynamic geological history which started with Litorina transgression (7800y BP) and is still ongoing.
As a consequence the coastline, which builds the interface between the brackish sea and the terrestrial surface and subsurface waters, has moved landwards".
by Maria Theresia Schafmeister, Univ Greifswald, Germany: Change of groundwater discharge os respone to varying climatic conditions-a model study at catchment svale at the Wismar bay/Baltic sea.
byw.cprm.gov.br/331GC/1343622.html

2. "the simplification coast to the line of seabanks described at Hill Pill, .....had two main effects: a gain in land for potentially year-round-use, and a shortening and smoothing of the line of a sea defence , although generally never to the extent that outfall works ceased to be protected in a recess from wave action".

to find this work you might type: Historical simplification coast

"Regular coastine" is the best I can find in the way of a technical term. It is contrasted with "irregular" or "indented coastline".

You can also find a fair number of examples of "uniform coastline".

Regular Coastlines
A Regular Coastline is smooth with very few natural harbors. Africa is an example of a regular Coastline. Regular coastline make building ports and harbors very difficult. Without these, trade, sea travel, and cultural diffusion are near to impossible. But, this also prevents invasion from the sea. This feature of Africa kept invaders out of sub-Sahara Africa for millennia.
http://regentsprep.org/Regents/global/themes/geography/bow.c...

Africa has the most regular coastline and, consequently, the lowest ratio of coastline to total area. Europe is the most irregular and indented and has by ...
http://www.britannica.com/EBchecked/topic/134805/continent

Lowland drainage systems are particularly susceptible to drowning as sea level rises, producing estuaries that severely indent otherwise regular coastlines (Plate C-23).
http://disc.sci.gsfc.nasa.gov/geomorphology/GEO_6/GEO_CHAPTE...

I assume that "simplification coast" is based on a German term, and it appears to be this:

Ausgleichsküste ... Translation, simplified coast, simplification coast, mature shore line, ...
http://dict.leo.org/forum/viewUnsolvedquery.php?idThread=263...

The signs are that people have had some difficulty finding a good equivalent for it in English.

I suggest "regular coastline" with some caution, because the German term have a more precise meaning within a specific system of terminology.

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Note added at 10 hrs (2009-11-21 00:17:24 GMT)
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... German term may have ...

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Note added at 19 hrs (2009-11-21 08:56:57 GMT)
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This link gives a good idea of some of the terminology used for describing coastlines:

http://www.coastalwiki.org/coastalwiki/FUTURECOAST_project,_...

You'll see that there's a term "soft coasts".

Frankly, I think it's all guesswork when it comes to finding a proper technical term. I've tried to suggest a "good enough" term for the general meaning, but I suspect that "simplification coast" - or the German term that lies behind it - has a more precise meaning (perhaps referring to the process by which the coast is shaped).