Sorry to bump an older thread, but thought I'd give an update. After I complained to GWR about this, and complained again about their initial response (which totally misunderstood the problem), they have actually done something!
To be totally transparent, GWR have been talking to me about this for a little while now, but I didn't want to say anything incase it didn't actually go anywhere.
The reason they have given is that indeed as some comments suggested, the fare was initially removed to "simplify the fares with the intention of making things easier for customers".
But long story short, I can't speak for all flows, but for at least some of the flows from the Bristol area to the South Wales valleys (I've checked my own regular journey and the journey mentioned in the OP), the Anytime return tickets that were removed from sale last year are now available again!
So thank you to GWR customer services, the fares team and anyone else who may have helped prod this along. I still think it is a bit of a joke we ended up in this situation in the first place (it has probably cost me a little under £70 extra over the 6 months the correct ticket wasn't available for), but at least the wrong has now been righted.