>>449467614mm f/2.8L II USM is the widest rectilinear lens Canon has ever made. It's very quite wide, well made, and sharp.
The two downsides I can think of is that it still has a bit of barrel distortion, yet not a significant amount, and the front lens cap is shit. You can buy aftermarket lens caps that are better for not a lot of money.
There's also the RF 14-35 f/4L IS USM which is more expensive, has more distortion, only fits RF-mount, and is f/4 instead of f/2.8. But, it has killer IS, can take normal front filters, is a zoom, and is just as wide as the EF prime.
A 10mm lens on FF has no hope of being rectilinear. To get a total image plane without fisheye levels of distortion would require a huge front element and would likely have a dogshit small aperture opening.
Now as an alternative, people have adapted the EF-S 10-18mm f/4-5.6 STM to full frame with a cheap metal mount conversion (I have done this) but you normally do it for video, as you will get heavy mechanical vignetting using a FF sensor size.
I bring this up because if you just want the widest lens that phyiscally fits your camera and you don't mind using APS-C mode, then this is a cheap and effective workaround.
