Widescreen resolutions are available to be selected when playing classic. The only option should be varying sizes of 4:3, but absolutely no higher than 1024x768.
That’s 100% not a bug. The Classic project differs from Vanilla as it is updating the game to run on modern hardware, while preserving the core game play and models from the original game. We get to play at higher resolutions and frame rates, we get lower pings, some graphics effects can be set higher, but the game is more or less the same otherwise in terms of numbers and models etc.