Yes. Assuming the current timetables are actually split (sometimes they're shown as split to cover EU rules). Portsmouth to Chichester or Bognor Regis every 30 minutes, Chichester or Bognor Regis to Littlehampton every 30 minutes then Littlehampton to Brighton every 12 minutes.It looks like this route is split. Is it possible to go by bus between the two?
The route has been split into three sections as Carl says in post #2 for some years now (since at least 2016). You may be able to hunt down some timetables in the web archive (archive.org) that will give you a precise date.
I believe the EU has nothing to do with the change, rather more to do with traffic congestion on such an immensely long route, and enabling resources to be better balanced (which is why there are more buses at the eastern end and a section in the middle than at the western end.
It is for similar reasons that the Kent & East Sussex end was split into sections. Reliability and resources.
In the late 80s the B&H bus was often an East Lancs bodied Scania decker, quite a strikingly modern bus in those days when everything else in Pompey was either a Transit minibus or, at best, a VR or mk 1 National.The 700 was historically a limited stop service in Southdown days with routes 230, 231 and 232, the idea being that you'd use the 700 to get near where you wanted to go, Chichester, for example, and then change to a 232 to go to your destination. It took 2 hours or so from Brighton to Portsmouth, after deregulation one run was operated by Brighton & Hove and the whole day's work was one return trip. If you were lucky they'd let you use a coach. It's a shadow of its former self now though, just like that horrid desert known as Pool Valley.