Removal of Off-Peak Day Returns, which are not regulated or protected fares, is an increasingly common tactic to offer customers a simplified fares structure in response to customer demand. It has also happened in the Welsh valleys.

Well I got my answer confirmed today by Merseyrail (not Northern Rail).
Merseyrail set the fare for the route and have removed off-peak tickets, despite not operating trains on the route. So it is roughly a 32% increase in the fare.
If I try Knutsford to almost any destination in Merseyside there isn't an Off Peak Day Return available and it's been that way for a number of years.
Although the strange one is not to a destination in Merseyside:
* Knutsford-Stalybridge only has Anytime Day tickets available for it.
* Knutsford-Huddersfield has Off Peak Day tickets, Anytime Day tickets, Off Peak tickets and Anytime tickets.
Passengers demand to pay higher fares. That's a novel perspective.
Knutsford-Stalybridge is priced by Northern, yet Knutsford-Huddersfield is priced by TPE which may explain the difference in ticket types available.