Yes there are some exceptions. There are some Non Commercial Guards and there are some DOO trains with staff selling tickets onboard. Like the example that you mentioned of Scotrail and also the SE OBMs and the SN OBSs too.
What i find interesting is that the Ticket Examiners on Scotrail will very commonly walk through the train and check and sell tickets. But yet the On Board Supervisors on Southern who perform the exact same job rarely ever walk through the train and check tickets and seem to mostly just sit down between stops and go on their phone or read the paper. It is very rare that a SN OBS checks tickets from my experience and normally if you want to buy one you have to go find them. So it is quite interesting how different TOCs have different attitudes to ticket checking.
Generally these days ticket checking is very rare. Even when i travel on trains with Commercial Guards or OBS or OBM onboard i find that SWR and GWR and SE and SN are all appalling at checking tickets. The staff on these four operators very rarely bother to check tickets.
SWR is a very odd one as they have a mixture of Commercial Guards (who check and sell tickets) and Non Commercial Guards (who do not check and sell tickets) but both types of Guards operate many of the same routes. So one day your train may have a Commercial Guard onboard and another day your train may have a Non Commercial Guard onboard. I have taken the same trains between the same places and it often seems quite random which type of Guard you will get.
Eurostar and Merseyrail are the only other operators with Non Commercial Guards but unlike SWR all of the Guards on Eurostar and Merseyrail are Non Commercial Guards and do not check or sell tickets.
I believe that C2C used to also have some Non Commercial Guards on their 12 coach trains (which they called "Train Captains" instead) until around 2018 or 2019 i think. I think the London Overground had Non Commercial Guards too until around 2013 or 2014 i believe.
So to summarise the current situation is:
Avanti West Coast, Caledonian Sleeper, Cross Country, East Midlands Railways, Grand Central, Hull Trains, Island Line, London North Eastern Railway, London North Western Railway, Northern, Trans Pennine Express, Translink Northern Ireland Railways, Transport For Wales, West Midlands Railway, all have Commercial Guards who sell tickets on 100% of their trains.
Eurostar, Merseyrail, both have Non Commercial Guards who do not sell tickets on 100% of their trains.
South Western Railway, have Guards on 100% of their trains but it is a mixture of Commercial Guards who sell tickets and Non Commercial Guards who do not sell tickets.
C2C, Great Northern, Heathrow Express, London Overground, Stansted Express, TFL Rail, Thameslink, are all fully DOO on 100% of their trains with the Driver being the one and only staff member onboard.
Chiltern Railways have Commercial Guards who sell tickets onboard 100% of trains North of Banbury and are fully DOO onboard 100% of trains South of Banbury with the Driver being the one and only staff member onboard.
Gatwick Express are 100% DOO with nobody other than the Driver onboard South of Gatwick and with an On Board Supervisor onboard North of Gatwick who sell tickets (but they are not guaranteed and the train can run without them due to staff shortages).