This setting defaults to false so there is no need to set it unless we
want it to be true.
This makes it easy to see at a glace which backends support this, and
matches the existing behaviour of other fields such as
`AllowAtAtStartOfIdentifier`, `AllowQuestionAtStartOfIdentifier`,
`UseAssignmentForEHBegin` and `AllowAtInName`. These are all only ever
set to true in subclasses, never false.
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