Recently I came across a user who is suspended:
Does this mean that the user is suspended until 2092? What type of offense might warrant such a long suspension, but not a permanent ban?
There is no option "permanent suspension" thus a very long suspension is used as a substitute for it.
The reason why there is no option "permanent suspension" is likely that it is too rare to be worth implementing and the workaround works well enough.
The maximal length of a standard ban is one year.
The user in question has said:
I do not participate on this site any longer, except to respond to comments regarding my own text, if that text is unavailable in another form. I do not accept the political moderation atmosphere here, it is not compatible with open science. Unfortunately, this seems to be a recurring pattern on such sites--- they grow with promises of open participation, and then shut down in a phase transition of censorious moderatorship. Hopefully physicsoverflow.org will be the first exception to this rule, as the policies there were crafted specifically to avoid this phenomenon.