Good post, even if dated, things are still very relevant.
MS is still using BOMs and UCS-2 for most things (including queries and strings in SQL Server!), Java uses UCS-2 for strings in memory. Rust uses UTF-8 as the main string type and can convert them to 32bit Unicode code points and back.
I sometimes get CP-1252 encoded events from Windows 2019 Server that I have to convert to UTF-8 before storing in logs DB! I also have lots of old emails encoded in who knows what sequence of encodings.
Plan 9 is all UTF-8 as it is where it was first created/used.
Most FLOSS is now using UTF-8 by default or is compatible.
Also same as with HTML the email body can contain encoding header, make sure it is sent before Subject header or things may not render correctly.
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