The Title::getSubjectPage() method is deprecated from version 1.34 and all places that call it should use the NamespaceInfo::getSubjectPage method instead. There are 7 usages in core and 8 in tests. Please note that the global change of this method resulted in some production errors: T227817
After removing the usage of getSubjectPage() from the core, we can check and fix also extensions (https://codesearch.wmcloud.org/things/?q=getSubjectPage), which will allow us to hard deprecate this method.
Change 932467 had a related patch set uploaded (by Barakat Ajadi; author: Barakat Ajadi):
[mediawiki/core@master] Title: The Title::getSubjectPage() method is deprecated from version 1.34 and all places that call it should use the NamespaceInfo::getSubjectPage method instead.
Change 932467 merged by jenkins-bot:
[mediawiki/core@master] Title: The Title::getSubjectPage() method is deprecated from version 1.34 and all places that call it should use the NamespaceInfo::getSubjectPage method instead.