I died to a ghast a super long way from my base in the nether. Luckily I dug a tunnel all the way there so it should be trivial to get it all. Then on the way back I dug out one tile (since one of the stairs was 2 tiles high) and the lava spread all the way down a long staircase and I died. Then I died again.
By the time I get back all my items are gone.
My heavily enchanted netherite pickaxe which I had finally gotten perfect (Fortune 2, unbreaking, mending, efficency 4), my good shovel (unbreaking+efficency 4), my not particularly good armor, the rest of my armor and tools, and all the spare niche tools I also had on me. I also lost my artifacts, (one gave 4 max health bonus, the other increased attack speed) which are randomly found in the world and can't ever be crafted.
Most importantly though is that all my backpacks despawned.
There were tons of stuff in them; at least 12 stacks of raw ores, backpack upgrades (worth 4 diamonds), 48 emeralds, stacks of logs and dirt and stone, as well as a ton of other stuff that I simply had in case of emergencies.
Aside from my enchanted pickaxe all of that is recoverable from given sufficient time.
But that isn't all. I also lost my mekanism bag, which contained every machine needed to upgrade from X3 ore multiplication to X4 ore multiplication. The big multiblock machine, the other two random but still quite expensive machines, the spare grinder and alloy infusers. Most importantly it lost me this:
A machine made using another machine made using another machine. It took a lot of miscellaneous resources, but most importantly it took 16(!!!!!) diamonds.
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I have a lot of stuff back at base, and I'll rebuild, and eventually I'll get another fortune pickaxe.
But ugh, it will take a while.
Fairly obviously modded minecraft
E: Just realized I was a complete idiot and realized I missed an entire item category: Infused diamond/redstone/whatever that would have let me use only 2 diamonds if I take a processing step in between instead of 16.
At least I don't need to mine a ton more diamonds.