The thing that comes up when I read this is that in most situations it will not work because you’re removing all the flexibility. The other thing is that most companies evaluate employees based on their availability and their flexibility.
There also a side note that employers are allowed to make different arrangements with employees.
Employers will probably adjust contracts from this point on that, if you’re in some sort of position where the availability is important, you’re obliged to answer which will render the law useless in a lot of situations.
There’s no one answer here, but it’s an important topic to think about.