In FAA-Land there are no contaminated runways, they dont get snow or ice or any other nasty stuff, therefore aircraft are not certified for operations on contaminated runways, the aircraft FLIGHT manual only contains certified data such as reduced thrust performance.
When airlines discovered that they were operating in a world that was very different to FAA-land, they insisted that the manufacturers gave them "SOME" data for operating on contaminated runways, the manufacturers decided as they weren't required to certify contaminated data, they would use the characteristics associated with contaminated runways that were developed by NASA using a Convair 880 aircraft. They then put this data as "ADVISORY" in the aircraft OPERATIONS MANUAL, which isn't a certified document.
The risk of operating on contaminated runways therefore lies with the airline and not the manufacturer. If any airline decided that they wanted contaminated data combined with assumed/flex and they had sufficient "Purchasing clout".... they could get the manufacturer to include that data.
But considering that a contaminated runway usually isnt 100% accurately reported, would you really want to take the additional risk of using Flex temperature?
Mutt