Now moved to the community Feedback and Suggestions space and unlocked again.
In answer to the question, there is no automatic removal of old discussions at present as far as I'm aware.
Some old discussions that were determined not to have been viewed for a long time were removed during the recent upgrade, but as far as I know that was a one-off process. I believe the admin are looking to set something up in the future that will archive off/remove old unviewed discussions after a period of time, though I'm sure that period will be quite lengthy (e.g. a few years old). In the meantime I wouldn't worry too much about it unless you have something really really valuable you want to keep.