The SAE levels don't really have exceptions like that:
https://www.sae.org/blog/sae-j3016-updateSome other quotes from wikipedia:
"Level 5 ("steering wheel optional"): No human intervention is required at all. An example would be a robotic vehicle that works on all kinds of surfaces, all over the world, all year around, in all weather conditions."
"Under all roadway and environmental conditions that can be managed by a human driver"
I've driven in all sorts of conditions - thick fog on the highway, dangerously heavy snowfall (no lanes), fresh snow without winter tires, etc. These things are not physically impossible they just have a higher probability of killing you. So what is a reasonable definition then? A brash, foolish, broke 19 year old? A stogy, conservative 70 year old? And how do you even quantify these risks, without actually taking a ton of risks, then walking back when bad things happen?
As long as there's someone out there willing to take more risks, they'll be more "capable" than an L5, thus L5s can never technically exist, since there'll always be more capable (but in actuality just plain risky) human drivers.