If there is an opportunity afforded by the TOC to join the train at a request stop by giving the correct signal to the driver or to disembark at a request stop by giving prior notification to the train crew member concerned, which is the case at many such stations (and the time for such a service to stop there is published in the timetable, usually with an "x" against the time), then these are valid stops on a route.
I am sure that the "request stop" station of Entwistle on the line between Blackburn and Bolton has been used in this quiz previously, as an example of this type of station.
Just look how many "request stops" are on the "Heart of Wales" line and on the "Cambrian Coast" line. You can book through journeys to any of these stations. They cannot take your money off you when purchasing a ticket to any of these, then refuse to let you disembark at the station that you have been issued with a ticket to do so.
The above is only my personal view of the situation as I see it and it requires someone with a deeper knowledge than I to confirm the exact situation