In my workshop I've been telling people "You can't delete a glyph using a substitution rule". The OT spec says as much: "A Multiple Substitution (MultipleSubst) subtable replaces a single glyph with more than one glyph". But... you can, Uniscribe, Harfbuzz and CoreText all support it, and Arial Unicode relies on the fact in its Devanagari implementation.

So it would be nice if there were syntax for it.