Super Mario Maker 2 is due out on June 12th, according to an Amazon UK listing. The listing simply says the game will be "in stock on June 12th, 2019". The page for the game is pretty barren otherwise. It's got some screenshots of Super Mario Maker 2 in action, as well as a shot of the game's box art.
Whether this release date information is a mistake or was uploaded in error remains to be seen. Amazon US lists the release date of Super Mario Maker 2 as December 31st, 2019, which is just a placeholder until the real date is announced. Other territories which would presumably get the game at the same time as the UK like France and Germany either don't have release dates or have the same December 31st listing as the UK one. We know that the game will definitely be out in June, but the exact date has yet to be confirmed by Nintendo.
Super Mario Maker 2 was originally announced for Nintendo Switch via Nintendo Direct back in February 2019. The game is a direct sequel to the Wii U's Super Mario Maker, and will allow players to create and play their own levels with assets from certain games in the Mario series. The four original game styles from Super Mario Maker will return, and players will also be able to make levels in a 2D version of the Wii U's Super Mario 3D World style. Super Mario Maker 2 will also give players access to new level features including slopes, modifiable auto-scroll, and a whole host of new enemies, items, and other level design tools.
Since the Wii U had no paid online subscription element, players could upload and play others' stages for free. We're not sure whether Super Mario Maker 2 will pick up that particular torch or whether a Nintendo Online subscription will be necessary. More details as we get them, as usual.