According to the code, it should still work:
From: XHtmlDoc.cpp in Sigil/src/BookManipulation/
const QString BREAK_TAG_SEARCH = "(<div>\\s*)?<hr\\s*class\\s*=\\s*\"[^\"]*(sigil_split_marker|sigilChapterBreak)[^\"]*\"\\s*/>(\\s*</div>)?";
So it should still work as either string is looked for.