The height limit below the bridge was reduced a few years ago. Not sure who decided but since then double deckers have effectively been prohibited - even the open toppers that used to run through when there was a route 43. The actual clearance to the road hasn't changed.
I believe the issue came after the road through Havenstreet was resurfaced. IIRC (as this must be around 2 decades ago now) the basic problem was that the old surface under the railway bridge wasn't removed before they slapped a new layer of tarmac on top. The next day when the road reopened several service buses - which had fit perfectly under the arch just a few days earlier - scraped their top corners.
Why the council or Southern Vectis didn't demand the contractors make good and return the road to its previous condition and advertised clearances I don't know, but after that, new (slightly lower) height warnings went up and that was the end of Havenstreet's hourly (or better) bus service. It couldn't support its own route without subsidy, and those subsidies dried up a few years later, leaving the village and station with no bus service at all.
All of this makes running a Rail Replacement Bus between Havenstreet and Ryde rather challenging.
(getting back on topic)
Apparently a new date for services starting has been mostly-confirmed. 1st November.
Your chance to travel on the 'new' trains is getting closer. South Western Railway have now announced the date for the reopening of the line
onthewight.com
Given where test running is at I'd say this is the most believable-looking date officially given so far. If it actually happens, then it will have taken a 10 month shutdown to achieve what was supposed to be done within 3. I know once everyone actually gets to ride it the delay will fade in our collective memory but it's not good.