Actually it would have been a contextual keyword, meaning it could be any word the committee wanted it to be, and it is only recognized as such in this particul

Actually it would have been a contextual keyword, meaning it could be any word the committee wanted it to be, and it is only recognized as such in this particular context (i.e. at the end of a function declaration). They could have chosen anything they wanted, but decided that '=delete' was fine. Other examples of contextual keywords are 'final' and 'override', which may also occur on function declarations.