My understanding is that this isn't really considered best practice any more, because those at the bottom are limited to only being able to do the work they are given.
I teach secondary so it is a bit different, but even in mixed ability groups, I will usually give everyone the same basic task, with extension and sometimes further extension work for those who complete it more quickly. It's not always the same students who use/need extension work, either- for example, if it's a more maths based task, some students will whizz through that and be ready for some real challenge, but with a more written task, they may not finish as quickly, or produce such high quality work, and it will be other students who need extension/challenge/whatever you want to call it questions.
We set from Y9, so after that point it's usually less of an issue, but I do keep banks of exam questions available for those who finish anything quickly!
But then at KS5, classes are essentially mixed again. I've got students in my A-level class who could get an A* and some who are working at an E grade. But obviously I don't want to limit any students. So, usually, all work from the same starting point, and then those who finish quickly AND to a high standard will be given additional work.