They're deprecated under the "Specification" tab: the `/icons/...` endpoints in the endpoint list, and the `icon` field in the `GridpointForecast` schema. The `shortForecast` field is documented as `A brief textual forecast summary` and not exhaustively enumerated, so it's also technically not machine readable. If it's anything like the icons, there could potentially be 50+ of these. I really couldn't find anything resembling an overall conditions enum or code anywhere, but I'd be happy to be wrong.

In the meanwhile, I could try to get it working with nearly everything else and try to find a last minute hack for the icons.