configuration options Syntax.ambiguity_enabled (inverse of former Syntax.ambiguity_is_error), Syntax.ambiguity_level (with Isar attribute "syntax_ambiguity_level"), Syntax.ambiguity_limit;
By chapter:
chapters 1 and 2, maybe 3: Peter White
chapters 6, 7 and 8: David von Oheim
chapter 6, maybe 8: Gerwin Klein
chapters 3, maybe 4: Tanja Vos
chapters 1-3 at least: Stefano Bistarelli