Before the 2.0 release of Chai, these assertions could be written as properties or as functions. For example:

expect(foo).to.be.true;
expect(foo).to.be.true();

expect(foo).to.be.empty;
expect(foo).to.be.empty();

expect(foo).to.exist;
expect(foo).to.exist();

expect(foo).to.be.null;
expect(foo).to.be.null();

The documentation on chaijs.com even explained that this dual-syntax was there to play nice with some IDEs and linting tools that flag the non-function syntax as an error. But in 2.0, the function syntax no longer exists. Was it removed by accident, or was there a reason for it?