In the past, there have been initiatives to convert previous versions of Bootstrap into SASS syntax, like https://github.com/twbs/bootstrap-sass for Bootstrap 3. However, as of now, I am not aware of any similar projects for Bootstrap 4.
Personally, I prefer using SASS over SCSS in my own projects. It's important to note that whether a library like Bootstrap utilizes SCSS or SASS doesn't really impact your ability to write files in SASS syntax and import variables from SCSS files. The beauty of SASS/SCSS lies in the flexibility it offers—just focus on writing your files in SASS without worrying about Bootstrap's choice of SCSS.