Maybe my formulation set some wrong expectations but the "new" API isn't going to be (and doesn't have to be) significantly different from the current one.
The only required extension to the API, mandated by the transition away from client side arrays, is the explicit specification and binding of vertex attributes within the shaders. Built-ins like gl_Vertex and gl_MultiTexCoord0 cease to exist in GLSL 3.00 along with the built-in matrix stacks (gl_ModelViewMatrix, gl_ProjectionMatrix, etc.).
A replacement mechanism for those is all that is needed to make the shader API "non-legacy". The rest of the current API can stay as it is. Uniforms haven't gone anywhere, and the compilation process has stayed mostly the same.
Sure... this won't encompass every addition that has been made to shaders in OpenGL up to 4.5, but those additions can be made iteratively on a case by case basis, like every other feature addition to SFML. None of it is necessary in contrast to what I described above.
