Is there a standard that regulates subject encoding in emails?

Question

Email

Is it obligatory by the standard to encode the subject in letters in the form
"=?UTF-8?B?VmFsZW50aW5hIEt1cmljaGV2YSDQtA==?=" ?
Sometimes I receive letters where the subject is simply in some encoding as in the example below, is this acceptable from the point of view of the standard?

Subject: ыЙОЩ ДМС чБЫЕК НБЫЙОЩ
Date: Tue, 29 Nov 2016 05:02:58 -0800
MIME-Version: 1.0
Content-Type: text/plain;
  format=flowed;
  charset="KOI8-R";
  reply-type=original
Content-Transfer-Encoding: 8bit

https://en.wikipedia.org/wiki/MIME#Encoded-Word says "Since RFC 2822, conforming message header names and values ​​should be ASCII characters; values ​​that contain non-ASCII data should use the MIME encoded-word syntax (RFC 2047) instead of a literal string"
at the same time as https://tools.ietf.org/html/rfc6532"However, fullinternationalization of electronic mail requires additional enhancements to allow the use of Unicode, including characters outside the ASCII repertoire, in mail addresses as well as direct use of Unicode in header fields like "From:", "To:", and " Subject:", without requiring the use of complex encoded-word constructs. This document specifies an enhancement to the Internet Message Format and to MIME that allows use of Unicode in mail addresses and most header field content."

Answer 1

Yes, I have. According to the basic standards (RFC 2822 / RFC 5322), email headers cannot contain non-ASCII characters.
There is an extension SMTPUTF8 (RFC 6531 / RFC 6532) that allows you to send UTF-8 headers without MIME encoding if your server advertises SMTPUTF8 in response to the EHLO command.
In all other cases, including the one you mentioned, the use of eight-bit characters in headers is a violation of current standards.