The warrior covert qre pro has excellent back/tailbone protection among pro stock pants.
the lower back guard is the thickest of any pro stock pant ive seen, at least an inch thick and very sturdy. then the tailbone is a curved one-piece pastic insert with foam, kind of like the shells they use for the hip except shaped to protect the tailbone.
the ccm hptk x or xp is also good. there is a nice thick segmented dense foam section for the tailbone. the only drawback is the section is quite narrow, so if your pants shifted during the fall you could miss it completely on impact. i will note that these pants are otherwise not super protective imo. relatively small hip shells, and not much padding except on the hip, front thighs and kidney/waist. i think the reason for their popularity among nhl players is their mobility--there is decent protection on high risk areas, but they are not overbuilt like the hp70 x (which, by comparison, has the largest hip shells of any pro stock pant, and hard shell protection on both the front and back of the thigh)
no pant is really designed to protect the tailbone if you fall straight downward though, like hitting the ice in an upright seated position. the padded shorts like tortoise pads would help somewhat with that type of fall, because the tailbone protection wraps around the glute rather than hanging behind. ideally, if you're falling backward, try to hit the ice with your whole back (but not your head). if youre really serious about injury prevention, learning a bit of judo would go a long way. obviously you cant control how you fall every time on the ice, but those judo guys are really good at controlling their falls so that they distribute their weight on impact.