Is there a standard governing kodirovanie subject-and in the letters?

Is required under the standard coding of the subject-and in the letters in
"=?UTF-8?B?VmFsZW50aW5hIEt1cmljaGV2YSDQtA==?=" ?
Sometimes I receive emails where the subject just in some encoding like in the example below, is it acceptable from the point of view of the standard?
Subject: yew LCA cbyc NBYO
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


In 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."
July 2nd 19 at 13:25
1 answer
July 2nd 19 at 13:27
Solution
Yes, there is. According to the basic standards (RFC 2822 / RFC 5322) e-mail headers cannot contain non-ASCII characters.

Is an extension SMTPUTF8 (RFC 6531 / 6532 RFC) which allows you to send headers in UTF-8 without MIME-encoding if your server announces SMTPUTF8 in response to EHLO.

In all other cases, including the mentioned, the use of eight-bit characters in the headers is a violation of applicable standards.
Thank you - alysson29 commented on July 2nd 19 at 13:30

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