In which year was Lego Dimensions discontinued?

Lego Dimensions was discontinued in 2017. Warner Bros. officially announced on October 23, 2017, that it would not develop further content for the toys-to-life game.

Released in 2015, Lego Dimensions combined a video game with physical Lego minifigures, vehicles, and a Toy Pad. Placing a compatible figure on the Toy Pad brought that character into the game, allowing players to combine franchises such as Lego, DC Comics, Doctor Who, Back to the Future, The Simpsons, and Sonic the Hedgehog.

The game received new content through a second year of releases, but a planned third year was cancelled. This is why some references describe the title as spanning 2015–2017, even though previously released game content and physical products may still have remained usable afterward.

The question asks when the game was discontinued, not when servers or every retail product disappeared. The canonical answer is therefore 2017, the year Warner Bros. ended further content development.