This thread was posted some time ago:

https://www.games2gether.com/endless-legend/forum/12-faq/thread/20162-have-shifters-have-not-seen-ice-in-60-hours-of-gameplay

But I've noticed the same thing. The prayer for ice shelves seems so rare as to be non-existent.

You can change winter settings to more harsh and you will see it more often. It shows up more often in the later winters.

Sure, but they almost never show up.  The final (turn 120+) winters in my latest game do not have ice shelf prayers, nor does it show up as a default effect ever. That seems unusually low.