What decides when a site is closed? I was guessing it was based on time elapsed since the last post there but there seem to be exceptions.
Also wonder about sites that say closed but later say reopened?
Topics are the actual name of what you’re referring to here as “sites”. Topics that carried over from the original site (Dec 2009 through May 2018) were closed if they’d been started prior to June 2017 approximately.
Some have been reopened by special request, in cases where there seemed to be benefit from a historical thread of useful information.
The general idea behind closing topics is that they tend to become more wide ranging and off topic the longer they stay open.