Keep the `!T` sigil?

Decided

D-0007, settled by the language’s author: part of spec §18. Older text cites it as 18.7.

No — ! moves to the signature. T? is a postfix type suffix; fallibility is marked !-> on the arrow and is never a type. Maybe<T> / Result<T,E> stay rejected: the value in a built-in is compiler-known layout and no instantiation in every signature, and that survives the notation change (§9)