A watford high street Travelcard is valid. This isn't even contentious! All booking engines will offer it.
Just because booking engines incorrectly interpret ticket validity, possibly due to incorrect spec, does not change the actual validity of a fare.
It isn't as if this is the first time it has happened either.
It is an outboundary Travelcard, as "in boundary" refers to Zones 1-6.
You are very much mistaken. Everything inside Zones 1-9 is
inboundary. There is no outboundary product valid for one return journey between the origin and the boundary of Zone 6 plus unlimited travel within Zones 1-6 if the origin is within Zones 7-9. The product is a Zones 1-9 Travelcard valid in Zones 1-9 only as per validity of any zonal product.
This is a TfL product and priced by them, as these were introduced when Zones 7, 8 and 9 were simplified from the original Zones A-D on the Met, where Travelcards from stations beyond Zone 6 had always been fully zonal since the introduction of Travelcards. TfL do not price any outboundary product.
The idea that creation of additional zones - designed to add validity - should result in a loss of validity is, frankly, absurd. It's also a very anti customer viewpoint but it's factually wrong too.
The validity of the Zones 1-9 Travelcard is widely advertised and widely known. It is a fully zonal product with no need to trace permitted routes between the origin and the boundary of Zone 6, as the origin is already within the validity of the Travelcard part. Starting from an origin inside the zones you cannot travel outside the zones in order to claim a "permitted route" on a paper ticket between two inboundary stations. Would you claim Slade Green to Crayford is permitted via Dartford on a Zones 1-6 Travelcard because it is valid on a paper point-to-point ticket? It's a ludicrous claim.
Whether there is any loss of validity due to the creation of Zones 7-9 is a separate and totally irrelevant point to the validity of the fare itself. You need to ask TfL if they considered this point when they brought stations on Watford DC Lines within those zones. Whether it is an anti-customer viewpoint is also irrelevant, and may I say unhelpful even, in a discussion about facts. I don't care what my view is, and indeed what anyone's view is, on what should and should not be valid, because that frankly means nothing in a discussion about what is and is not valid.
I am sorry but I find it grossly irresponsible to suggest that the validity of the Zones 1-9 Travelcard extends to a station outside these zones. While it may be the case that anyone armed with such an incorrectly generated itinerary should see the itinerary honoured, it does not mean this fare is officially valid at Watford Junction. Such a suggestion could land an inexperienced customer in very serious trouble with a big financial penalty.
If you could find me any official documentation unrelated to journey planning specs confirming that a Zones 1-9 Travelcard is valid at any station (any will do) outside these zones then I will happily accept your interpretation.