With help from several volunteers for Reddit and several of my Steam friends, who helped me gather screenshots, I've collected and analyzed data on 860 chests to figure out the chances of getting each item type and rarity, with Plunderer's Instinct perk (390 chests) and without it (450 chests). We also used Ace In The Hole perk to get similar data for the add-ons it generates.

Edit: Since writing this guide, I have added this information to the game's Wiki. Please don't be one of the misinformed people in the comments who say my work is pointless because the information was already on the Wiki. Thank you.

If you like my work, please upvote this guide to make it more visible and promoted in the guides list.

Here is the original Reddit thread I posted a month ago, specifically the comment that contains links to the full table with the raw data, packs of the screenshots if anyone wishes to confirm the authenticity of the data, and extra comments/considerations about the results.

Disclaimer: my Steam and Reddit usernames are different, but I'm the same person.

I have also made a similar guide about chances of getting items in the bloodweb according to your Prestige tier: How Prestiging affects item rarity (with data from 240 bloodwebs).

Considering how many sheetpost guides there are for this game, I cannot stress the importance of giving this a guide a thumbs up show up as a suggested guide for other people. Thank you very much for upvoting!