It is mentionned on the ECMAScript standard here that :
... 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.
There is also a red warning on MDN : String.prototype.substr() MDN doc
Does anyone know why (ECMAScript standard say that) programmers should not use or assume the existence of String.prototype.substr ?
Because it's never been part of the standardized language. It wasn't in the ECMAScript 1 or 2 specs at all, and only appears in ECMAScript 3 in Section B.2 ("Additional Properties") (and subsequent editions in similar annexes), which said:
Some implementations of ECMAScript have included additional properties for some of the standard native objects. This non-normative annex suggests uniform semantics for such properties without making the properties or their semantics part of this standard.
Moreover, substr is largely redundant with substring, but the second argument has a different meaning, as well as with slice.
In pragmatic terms, I'd be surprised if you found a mainstream JavaScript engine that didn't provide it.
来源：https://stackoverflow.com/questions/52640271/why-string-prototype-substr-seems-to-be-deprecated