Depends on the medium. EPUB 2.0 (and later revisions) specifically supports vertical RTL text for use-cases like Japanese novels. Additionally, many novel reading websites support toggling between vertical and horizontal text. Vertical text implicitly switches to RTL text direction.
Of course, this is not a general use case. But saying "modern Japanese is LTR" is not quite accurate. Computer / digital media is commonly LTR and horizontal, but a single step outside exposes one to vertical text, and RTL text in physical newspapers, literature, comics, a subset of textbooks, and handwritten signs that try to look "traditional" in style.
Depends on the medium. EPUB 2.0 (and later revisions) specifically supports vertical RTL text for use-cases like Japanese novels. Additionally, many novel reading websites support toggling between vertical and horizontal text. Vertical text implicitly switches to RTL text direction.
Of course, this is not a general use case. But saying "modern Japanese is LTR" is not quite accurate. Computer / digital media is commonly LTR and horizontal, but a single step outside exposes one to vertical text, and RTL text in physical newspapers, literature, comics, a subset of textbooks, and handwritten signs that try to look "traditional" in style.