Sitting for I’tikaaf in the musjid during the last ten days of Ramadhaan is an ibaadah which is sunnah mu’akkadah alal-kifaayah. (Durrul Mukhtaar 2/442)

Sunnah mu’akkadah alal-kifaayah refers to a sunnah whose obligation is placed upon the entire locality collectively. If one or a few individuals of the locality fulfil it, the entire locality will be absolved of the obligation. However, if no one from the entire locality fulfils it then all the people of the locality will be sinful for neglecting it.