Very few people do the accompany tract. Very few pianists are willing to do accompaniments, in general. It competes with teaching in the thank-less stakes. Few people are willing to do specialized training just so that they can be heard and never seen.
You don't even get your name of the candidate's certificate when you do it. Seeing as most people get into the performing arts to get some sort of validation for there egos, it stands to reason that something as completely lacking in glory, as accompaniments, will not be popular.
- Everyone taking Grade 5-8 will also have to take the piano accompanying exam?
Nobody is forced to take these exams, it may lead to you being a better pianist, it may even open some doors for you, but no one is forced to do it.
- After Grade 4, the do tracks separate into Piano Solo and Piano Accompanying?
The tracts were never together really, you are just expected to have basic proficiency before you begin the accompaniment tract. You can very easily have Grade 8 - Accompaniment be the first exam you play.
What do people generally do?
usually, if they can help it they don't do this.