It’s available now. Hold the ChatGPT’s icon, and you’ll see ‘Voice’; hold it down and drag it to the home screen. Voila! One tap to talk.
Note: I’m using a Pixel, so maybe this is some kind of Vanilla Android thing (if it doesn’t work for you).
Now, the most interesting part: you can set Quick Tap (double-tap at the back, again only on Pixel) to open this shortcut more quickly. Just double-tap, and it will instantly open up ‘Voice’ shortcut.