Are there any free API for cc 0 SVG illustrations? something like undraw but with API service too.

I haven't come across any free APIs specifically for CC0 SVG illustrations, but you could try exploring resources like Art4youSpace. I've recently found great woodland creatures SVG files https://www.art4youspace.net/col... there. Just be mindful of the respective licenses and attribution requirements. Alternatively, you could consider commissioning custom SVG illustrations.