There's at least three plausible answers to the one question - and, even then, all are open to interpretation:-
1. The general (international) consensus on the term 'MODERN' Soul is any production post the 'CLASSIC' or 'GOLDEN' era of Soul Music (which, pretty much everyone agrees, was the sixties).
2. From a 'purist' perspective it's a term which can only be applied in a 'literal' sense to 'Current' soul music. I feel that (although it's a fair description of 'New' Soul Music) that approach is extremely limiting, almost impossible to build a 'scene' around and is consequently a minority (though perfectly accurate) viewpoint.
3. In a 'Northern Soul' context 'MODERN' Soul is, as Steve M describes, an offshoot (post Wigan) of the Northern scene, championed, largely, by Sam & Arthur - and aided and abetted by others!
That said, nobody with any knowledge of Soul Music (in a UK Northern 'scene' context) could deny that such Post 'GOLDEN era' classics as Esther Phillips, Rodger Collins, Ernie Bush, Sisters Sledge etc were (at the time of release) every bit 'Northern Soul Scene' monsters... and should, therefore, be just as relevant to the 'Northern Scene' as, say, the Tomangoes, the Incredibles or Freddie Chavez...
... but only as oldies!
Personally, I've always felt that those who favour music from all eras are truly fortunate. Who wouldn't want to be able to obtain the same amount of pleasure from 40-50 years of great music, as opposed to just the music from one decade?
However, if individuals are happy to miss out (as so many seem to be) then good luck to em.
Sean