Honestly plaintext has a pretty bad record as a format, with all of the UTF-16, Latin-2, Shift JIS, and so on. I’d suggest paper, or for true archival storage, parchment and stone.
I'd suggest an unbroken chain of oral transmission...
Much more slowly and gracefully than any digital medium we have concocted so far (save for core rope memory, maybe).
> even if preserved can become unreadable (e.g. Linear B) without a surviving linguistic community [...] translations produced with century-scale gaps often lose subtext
This pertains to the message and not the support; also, I'll take missing subtext over missing text any day of the week, thank you very much.
https://arcticworldarchive.org/ would like a word
Furthermore, the vault contents are stored on piqlFilm, which in turn uses Boxing barcodes [1]. So, they literally rely on an analog support.
I restm my case.