Is there any type of flash-based storage (preferably accessible to end users) that focuses on long term data retention?

If not, that feels like a substantial hole in the market. Non-flash durable storage tend to be annoying or impractical for day to day use. I want to be able to find a 25 year old SD card hiding in some crevice and unearth an unintentional time capsule, much like how one can pick up 20+ year old MiniDiscs and be able to play the last thing their former owners recorded to them perfectly.

Yes, NOR Flash guarantees 20 years data retention. It's about $30/GiB.

There are also expensive high-grade SLC NAND flash chips available that offer significantly higher retention time than the cheaper commodity channel TLC NAND (i.e. 10 years versus 3 years).

In general, though, whether NAND or NOR, the fundamental way that flash works is by creating an isolated voltage charge for each bit (or several bits, for TLC), making it effectively a vast grid of very tiny batteries.

Like all batteries, no matter how well-stored, they will eventually leak energy to the point where the voltage levels change enough to matter.

Further, it's not enough to simply make power available to this grid since the refresh of the cells requires active management by a nand controller chip and its associated software stack.