It's a good practice to maintain a list/smartlist of people you wish to have in your red list and always use that as a suppression in all your campaigns. This keeps things simple!
It's a useful practice to have a smartlist with criteria for your unmarketable leads and do a suppression using that as well.
While true, you can't enforce the use of a suppression list. So it's important that the behavior of the system fields be understood.
In any case, this thread is 8 years old! So not really an active source of best-practice info.