https://wccftech.com/galaxy-s21-ultra-case-renders-show-how-the-s-pen-will-be-integrated/

We are only 3 days away from the official unveiling of the Galaxy S21 series, and by now, we are aware of everything that the devices are going to be. The star of this is how it will be the Galaxy S21 Ultra, and that's for several reasons. Not only is it the most expensive, but it is also going to be the first Galaxy S smartphone that will have full support for the S-Pen.

However, the S-Pen will be a separate accessory and will not be housed within the phone as there is no slot. This generally means that the first and third-party case manufacturers will have to come out with their own way of ensuring that the S-Pen has a place inside the case.