Is slicer free for the user? Like if after some times it shouldn’t say to upgrade to pro for better results and all?
Indeed it is quite unbelievable, but it is free. Not only free, it is also open-source meaning all the source code is available:
There is no ‘pro’ version to buy even if you wanted to.
Also, it comes with a non-restrictive (BSD-type) license, which allows using the entire application or parts of it in commercial products without paying anything to the developers.
More than $50 million have been spent on development of 3D Slicer, but we can make the software available for free, because the work is funded from research grants (primarily by US National Institutes of Health) and from some contribution of private companies. A selection of major funding sources are listed in the acknowledgments page - 9 of them are currently active.