Seems that this was thought to be an issue for Brown to play in Denver, but apparently it is not a problem in his case. There was discussion last year that he might sign in Denver, and apparently health would not be an issue to play there.
https://thednvr.com/source-close-to-john-brown-says-sickle-cell-trait-wont-keep-him-out-of-denver-doctor-weighs-in/