As I said in
my previous post, Trainline are not the only ones offering Day Returns (together with period returns) after the passenger initially selects "open return" - the main problem is that their "SplitSave" logo hides the words "Day Return", and that the cheapest ticket is shown separately above the "Return same day"/"Return within 1 month" sections.
Attached are screenshots showing Day Returns being offered after "Open Return" is selected, from the LNER app, the Crosscountry app, the Scotrail app, the GWR app and the c2c app (which I think covers a range of various parent companies). The only app I tested whose "Open Return" didn't include Day Returns was Thameslink (so presumably the other Govia TOCs as well), where if you want a Day Return you have to plan a return itinerary.
So perhaps The Railway has actually decided that "Open Return" means what Trainline thinks it means, and to be honest I think it makes much more sense than the old definition. As long as "Day Return" is prominently shown at the ticket selection phase I don't see a problem with it.