> One logical possibility would be written records of some sort, using a substrate that hasn't survived—prone to waterlogging or insects, modern paper would certainly not survive across tens of kiloyears

Sometimes impermanence is a feature, not a bug. I was always told by my elders: 'The Internet is forever'. Once you put things on it, it's hard for them to be removed. A permanent record of sorts. But over the years I've found many sites just evaporate with entire Library of Alexandria events occurring daily on the web. Yes, we have Archive.org which has mirror copies, but it doesn't catch everything.

And bit rot is useful too. After I die, I don't really want my precious data lingering around. Let it rot.