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I am an amateur builder (it's a weird hobby), having built two houses from scratch and many extensions. Here's how it works:
The answer to the original question is yes, new builds are still using cavity insulation and traditional rockwool/ fibreglass in the roof space. The only real change in the last few years is that new builds require the floor to be insulated, usually under the screed.
Cavity walls were introduced in the c1940s, as a solution for damp, not for insulation. The standard was, and still is to a degree, 50mm gap between the 'leaves'. Wall ties are used to tie the 'leaves' together. These were then altered to incorporate a 'drip' in the centre (usually a bend in the tie) to prevent moisture tracking along the tie to the inner leaf.
Cavity insulation was adopted in the c1960s. This involves cavity 'batts' (standard size sheets of rockwool/ fibreglass insulation) laid against the inner leaf of the masonry with an air gap of at least 20mm to the outer leaf; to prevent moisture tracking. The wall ties were then fitted with a grip to hold the batts in place and stop them falling into a big heap.
In the last 20 years or so, solid cavity insulation batts have become available, these are 'celotex' type, ie solid foam with foil backing. They give much higher insulation values but are much more expensive than traditional rockwool.
One problem with cavity insulation is that bricklayers always fit it, as it goes in as the masonry is laid. Bricklayers are always on piece work so faster makes more money. They also have little knowledge of what the insulation is required to do so don't install it very well. Once finished no-one is ever going to see it again, either.
Retrofitting cavity insulation, ie drilling a few holes at various points and 'squirting' some kind of insulating material (be it poly beads, foam, even paper pulp), is not good, as it will fully fill the cavity, there is no way of it not, and give moisture an easy path to track from outside to inside.
Insulation is good, moisture is bad in a building. The hard part is doing one properly and effectively without the other being a problem. It is quite simple to achieve on a new build if you understand it and install everything correctly. The last two parts are where the problems can lie, as housebuilders are not known for their attention to detail.
Soffits and fascias are just one way of venting a roof void. There are others (ridge vents is one of many) and generally depend on design of the building which are utilised.
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