I have fished for over 10 full Minecraft days (not nights) on a vanilla MC servers (Java) and have not gotten a single Nautilus shell. I am fishing in warm ocean from the shore.
I have gotten over 10 of each of the other 6 "treasure" items. I am using a Mending, Luck of the Sea III, Lure III, and Unbreaking III rod.
I will continue try, but over this period of time, something is definitely not right.
Further information.
Nothing in the official Wiki indicates that there is any difference in location or proximity to structures.
Both were in a lukewarm ocean, but one of them was within sight of an Ocean Monument.
When I moved to a mesa biome on the coast of a lukewarm ocean, I got 5 within 1.5 minecraft days of fishing.
If this does make a difference, the Wiki needs to be updated accordingly.