It is mentioned 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 through to today [ES2022 draft as of this writing]), which said:¹
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 and slice, but the second argument has a different meaning.
In pragmatic terms, I'd be surprised if you found a full mainstream JavaScript engine that didn't provide it; but I wouldn't be surprised if JavaScript engines targeted at embedded/constrained environments use didn't provide it.
¹ That wording has changed more recently to:
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 ECMAScript hosts. 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 define 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.