[Erolast]

помню свойство __proto__ вынули из раздела deprecated и стандартизировали в ES6.

До ES6 его вообще не было (http://www.ecma-international.org/ecma-262/5.1/, ctrl+f), в ES6 его добавили сразу как legacy.

Annex B (normative) Additional ECMAScript Features for Web Browsers

The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.

NOTE This annex describes various legacy features and other characteristics of web browser based ECMAScript implementations. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex defined the requirements for interoperable implementations of these legacy features.

These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.

http://www.ecma-international.org/ec...r-web-browsers

__proto__ входит туда же, под индексом "B.2.2.1" - http://www.ecma-international.org/ec...type.__proto__

То есть, юзать, конечно же, можно, но, во-первых, отличные от браузеров окружения не обязаны его поддерживать, а во-вторых, это прямо не рекомендуется в спецификации (а значит, в один прекрасный момент может быть запрещено в каком-нибудь 2016 strict mode).