There are three different interpretations for the term "open return" these days.
Firstly, the former Standard/First Open Return (SOR/FOR) ticket type, as used by British Rail and the early years of privatisation, has been called an "Anytime Return" since 2008, retaining the SOR/FOR codes to this day. However, these days, relatively few people us the term Open Return as exclusively referring to an SOR/FOR product.
Secondly, a significant proportion number of people refer to the term "Open return" colloquially to refer to any product (including one with time restrictions) that is valid for a calendar month on the return portion, i.e. the return portion has the same duration as the traditional Open Return product.
And finally, there is a third definition of the term "open return" which is that referred to by many websites, including Trainline from as far back as around 1999, which can be a time restricted ticket and doesn't even have to be valid for more than a day, but is not an "Advance"; colloquially this is more commonly known as a "walk-up" ticket, or occasionally as a "flexible" or "semi-flexible" ticket, and can be almost any type of ticket that isn't restricted to one specific service/itinerary.
Therefore, it might be a good idea to avoid using the term "open return", as the meaning can vary by context, as well as being subject to interpretation, as there is no commonly accepted definition, and it's no longer the name of a product.