For me it’s less about claustrophobia and all about heat. I burn too easily to have the top down in spring/summer and with the top up it’s a torture box. When the window is lowered, it acts more like a nice shade awning than a sealed roof.
I have two boot covers, an aftermarket item without the slots and an original with, which is what I use. The original cover came from a 1969 OTS which, as far as I’m aware, was the first year the top boots featured the slot but earlier than the debut of the inertia reel. So, to answer your question, the slots were made to accommodate the static belts and are in the right place. The main benefit of the slot is it reduces the length of the shoulder strap but it also keeps the strap from falling behind the seat back when you exit the car. This is how it fits my ‘68:
It’s a greenish-grey, as Doug says. As far as I know the colour remained approximately the same for decades. I have the original, unrestored soft top frame from my XK120 and the colour is the same as the soft top frame of my ‘68 E. When I stripped and resprayed the E-type frame back in the 80s I acquired a pint of acrylic lacquer based paint from British Auto USA, “Jaguar Seat and Top Frame Paint”, their p/n was CH2006. British Auto USA was of course bought out and became SNG Barrett USA, who up until 10 years ago at least listed the paint under p/n CH2006/3. I’ve just checked and it seems it’s no longer listed, but perhaps it just hasn’t made it to their on-line catalogue.