It's actually been the default since v1.55 which released early April 2021: https://code.visualstudio.com/updates/v1_55#_webgl-renderer-...
Before that from v1.17 (~October 2017) it was using a 2d canvas context: https://code.visualstudio.com/blogs/2017/10/03/terminal-rend...
> GPU acceleration driven by the WebGL renderer is enabled in the terminal by default. This helps the terminal work faster and display at a high FPS by significantly reducing the time the CPU spends rendering each frame
https://code.visualstudio.com/docs/terminal/appearance#_gpu-...