64GB and 512GB LCD will be as long as stocks last, presumably along with refurbished units of the same. How long that will be is impossible to predict without more info on what sort of stocks they have and how many are being bought.

256GB will continue to be available for the foreseeable future as it’s still in active production as the base model. That £349/$399 price point seems to be something Valve values. If over the next, say, year or so, they can get the price down on the OLED, I wouldn’t be surprised if before the next holiday season they do another, smaller refresh of the lineup, maybe bringing OLED to the base models and bumping the storage again.