The product page at B&H Photo for Sony's digital writing slate now describes the device as "DISCONTINUED" is bright red text, and what with new units no longer available on the related Amazon page it would be reasonable to conclude that it is really off the market.
Originally launched in Japan in 2014, due to its high price and limited feature set the DPT-S1 has had little success in the market. It originally cost over $1,000, but the price was gradually reduced to $699.
Give that the DPT-S1 had an expensive screen, two touchscreens, and all usual ereader guts, that was a good value But Sony did little to repair the software shortcomings. This device only supported PDF, and its software suite never got much further than editing PDFs, browsing with a limited web browser, and syncing documents with the cloud.
With the DPT-S1 gone, the only remaining plus-sized reading device is the Onyx Boox Max. (While Pocketbook has their own 13.3" device in the works, there's no evidence to suggest it will ever be released.)
Some might hope that Sony will release a new model, possibly called the DPT-S2 and include the new 13.3" E-ink screen, but that screen is nowhere near ready and anyway there's not even a hint that Sony is interested in a new model.