First of all, I believe that for serious offenses such as scams etc. we should have a way to exclude such members from the forum.
But, what about some other, no so clear cases like plagiarism?
No matter due to plagiarism or other reasons that led to permanent bans, those users need to have significant positive net-effects. Perma-ban appeal will be handled case-by-case, but only cases which have good net-effects will see perman-ban removed.
There's been no policy change. redsn0w wasn't permanently banned due to several factors which made me think that permabanning him would be a net negative for the forum. Nobody is banned strictly because of "the rules"; it's always handled case-by-case, but almost always, plagiarists deserve to be permabanned.
If you think that a ban should be ended, make your case in a new topic from a "good for the forum as a whole" perspective.
A few people have their bans removed for numerous reasons. It's not plagiarism if it's your own work and if you can prove that then the ban should be lifted in those cases.
For plagiarism, please read there:
[TIPS] to avoid plagiarism