Unfortunately BR Fares doesn't show where e-tickets are available; this data comes from a separate feed (RCS) to the fares data. E-ticket availability remains a complete hotch-potch for now, sometimes even varying on a particular journey based on the ticket type! The only publicly available method to check it is to use a retailer that you know supports e-tickets, and then see whether it offers you them.
Just to add to the complexity, note that Trainline (which includes their whitelabel clients, such as XC, GA, EMR, WMT and ScotRail) will sometimes offer an e-ticket where other retailers can't. This is because they are de facto exempt from using RCS data and use their own data instead.
E-ticket availability is continuing to improve, with Southeastern and ScotRail both being in the process of rolling them out more widely as station barriers and staff ticket checking facilities are upgraded. Unfortunately I imagine Merseyrail, TfL and TfW will remain hold-outs for a long time to come, as they aren't under DfT control and like to do their own thing.