Yep. Your best bet is to ask for "solid white/black background" and then feed it into something like rembg [1]. It's an extra step but it'll get you partly there.

On the OpenAI side, the gpt-image-1 model has actually had the ability to produce true alpha transparent images for a while now. Too bad quality-wise they're lagging pretty badly behind other models.

[1] - https://github.com/danielgatis/rembg