Decommissioning large facilities can be very different from site to site.
Some are of course unplanned but there’s also the opposite: decommissiong is planned well in advance. One case is the observatories atop Mauna Kea, involving several countries, a mix of US universities and privately funded telescopes, a sensitive ecology, a long history of non-US presence and sometimes strong protests against the large scale facilities from environmental activists and Hawaiians protesting against the presence of big observatories on this “sacred landscape”.
Perhaps because of this contested nature, meticulous decommissioning planning goes on well in advance of the actual shutdown of these telescopes, as can be seen in the Decommissioning plan for the Mauna Kea observatories (pdf).
