Northern Soul came to life as a youth cult: a 'scene' made up of like-minded people with shared musical tastes. Now it is a 'mature scene' of occasionally like-mined people who still have a love for aspects of the same music. Many have become cantankerous with age and set in their ways. For these people the 'scene' is no longer alive. Instead it is a thing of nostalgic beauty, full of warmth and happy memories that allow them to revisit their youth over and over again.
For others, the 'scene' has never stopped being alive. For them it is a scene that, even 40 years later, continues to develop.
Northern Soul is and always has been a cultural phenomenon, not a musical one. This is because Northern Soul does not exist as a musical form and never has done. No artist or group ever set out in the 60s to record a 'Northern Soul' song. Apart from a handful of spurious tailor-made recordings the same is true for the 70s.
The question of "what is Northern Soul music' thus cannot be answered. Instead, the question should be "what sort of music is played and listened to by people on the Northern Soul Scene?"
That would give you a much more sensible answer. It would also reduce the possibilities for the type of fractious opinions that this type of discussion always creates.