Some setttings may have characters 'in a box' more than others; and one should not always assume they can 'refluff' everything without some discussion with the GM... but anything outside of the 'clearly rules' part of the text is background and guidance, not rules unless the GM says otherwise (and this is ok if they do for setting or story reasons)

Do RP rules still exist? A few... Paladin Oaths, Cleirc Gods via divine intervention, Druid armor restrictions, some mechanical aspects of alignment (particularly of magical alignment change) and mind-influencing Magic... but they are few and far between, and the 'flavor text' of class descriptions isn't (in my opinion) anywhere close to binding in that way