I think it was done that way intentionally, so it's simply a case of perldelta being a bit imprecise.

The reason for escaping { is to detect problems with improperly coded {} quantifier, and to allow \X to be extended to \X{...}. (For example, there is now \b, \b{gcb}, \b{wb} and \b{sb}.) It's not likely a bug to have a leading {, and it can't be a \X argument, so there's no real reason to deprecate it.