My general experience in recent months is that on five-car 800s operating on their own, they have cracked the seat reservation issue and I can't remember the last one I rode on when the traffic lights system was not working. Nine-car sets they don't seem to have got the software fixed yet, so card reservations still the norm.
I can't speak for coupled pairs of five-car 800s, since I hardly ever ever use them, nor for 802s as I've only had a couple of rides on 802101 and it seemed to be in the same boat as the nine-car 800s, with card reservations in use both times.
HSTs should obviously still be card reservations but if the printers are broken/someone forget to pick up/put out the cards, etc (which could happen in the past as well, it's hardly a new thing) then there may not be any.
Coach F is the one that is left unreserved in standard on HSTs. On long IET formations it should be coach G.
Old FGW HST seating plans are here - the West Country services should be the low-density layout but with the fleet being run down, someone else may know whether this is still the case or whether some high-density coaches have infiltrated sets now. First class is now lettered K and L as per the more recent GWR plan at the second link below but that one only shows a high-density formation.
http://ehelp.thetrainline.com/euf/assets/images/seating-plan-firstgreatwestern.pdf
https://www.gwr.com/~/media/gwr/pdfs/safety-information/hst-seating-plan.pdf