Instagram should able to handle sharing ProRAW photos as of this morning, may take a bit to propagate out.
Apple says that ProRAW combines the information of a standard RAW format along with iPhone image processing, which gives you more flexibility when editing the exposure, color, and white balance in your photo. With iOS 14.3 or later, your iPhone 12 Pro and iPhone 12 Pro Max can capture images in ProRAW format using any of its cameras, including when also using the Smart HDR, Deep Fusion, or Night mode features.
Notably, when asked if the Instagram editing tools take advantage of the format, Johnsen replied, "They do not, highly recommend other editing apps like @usedarkroom for that."
Darkroom was just updated with ProRAW support yesterday.
You can download Instagram from the App Store for free.