Earlier quoted context omitted.
Purely anecdotal, but I hoard a lot of personal documents (shopping receipts, confirmation emails, scans etc.) and for stuff I saved only 10 years ago, the toughest to reopen are the pure text files. You rightly mention Unicode, as before that there was a jungle of formats. I have some in UTF-16, some in SJIS, a ton in EUC, other were already utf-8, many don't have a BOM. I could try each encoding and see what works…
> stuff I saved only 10 years ago There have been a lot of practical options around in the last three decades for using Unicode. To name just a few: Unicode is around since 1991. UTF-16 was supported in Windows NT in 1993. XML (1998) was specified based on Unicode code points. ...
Half the mail I received from that period was in iso-2022 (a JIS variant), most of the rest was latin-1. I have an auto-generated mail from google plus(!) from 2015 in iso-2022-jp, I actually wonder when Google decided it was safe to fully move to utf-8.