The [0] change arbitrarily modified the visibility of the mCategoryViewerClass property which in turn breaks extensions [1, 2] that rely on the existing visibility.
As for comparison, REL1_29 passes [3] all unit and integration tests while the MW master branch [2] returns with:
PHP Fatal error: Access level to SIL\Category\LanguageFilterCategoryPage::$mCategoryViewerClass must be public (as in class CategoryPage) in /home/travis/build/SemanticMediaWiki/mw/extensions/SemanticInterlanguageLinks/src/Category/LanguageFilterCategoryPage.php on line 18
The argument in T166483 is understood but changing the visibility is not a solution as one cannot catch the issue while trying to maintain compatibility [4] with REL1_27.
I'd rather see that a new public method is introduced (e.g.CategoryPage::getCategoryViewerClass) which would allow subsequent implementations to override the return value without the need to modify the CategoryPage::mCategoryViewerClass property.
[0] https://github.com/wikimedia/mediawiki/commit/17d5e809d9e64edd1eeebcad018b068ca5b86e17
[1] https://github.com/SemanticMediaWiki/SemanticInterlanguageLinks/blob/1.3.0/src/Category/ByLanguageCategoryPage.php#L21-L24
[2] https://travis-ci.org/SemanticMediaWiki/SemanticInterlanguageLinks/jobs/258804178
[3] https://travis-ci.org/SemanticMediaWiki/SemanticInterlanguageLinks/jobs/258804176
[4] https://www.mediawiki.org/wiki/Deprecation