Two thoughts based based on @etr221’s comment
1/ I recall there being an ongoing pair of group action lawsuits aimed at train operators and other ticket sellers not making Boundary fares explicitly available through all sales channels (I think GTR was one of the operators being sued).
And websites don’t always allow boundary tickets to be bought, and certainly don’t do it in a consistent way. A spot check of a few: Nationalrail, thetrainline, Thameslink: NO. GWR: yes, search for zone/underground, Greateranglia yes zone/travelcard.
Note none of these prompt with anything useful if you put “boundary” as the search. Not at all satisfactory.
2/ @Mike seems like a lot of the questions on oysterfares forums recently are dealing with extension fares/edge cases for season travelcards/60+/zip/gold, etc. I wonder if the fare finder could start highlighting what is needed and where to buy for the more common examples e.g. the ends of the Elizabeth line and other special fare zones (contactless not oyster, Gatwick/Luton/Southend airports) , and maybe a “flowchart”/”Wizzard” type experience for boundary fares on national rail more generally.