This was added in https://gerrit.wikimedia.org/r/c/mediawiki/core/+/866668 as a stopgap fix for T324890, T324891, T324901 but that usage is inconsistent with plans for async rendering and the Parser->Parsoid migration and should be reverted. Ideally we could add some sort of wfDeprecated() warning to handle this with our usual deprecation mechanism -- maybe by implementing the __get() magic method.
Communication between extension tag invocations on the same article page should in general be discouraged as it disallows independent/asynchronous rendering & caching. Often coordination between fragments can be done via a post processing hook once all the fragments are rendered. Where coordination is "necessary" it should be done via serializable data in the ParserOutput object, not via the Parser object.