A: Middlesex was abolished as a county in 1965 for both "administrative" and "non-administrative" —i.e. all— purposes. Following WP:UKCOUNTIES we can either say Middlesex was a county or Middlesex is a former county. The second wording makes more sense because it is still an area name, if no longer a functioning county.
Q: But it is still used in postal addresses?
A: Royal Mail no longer use or recommend the use of a postal county in addresses and have changed their nomenclature to 'former postal county', and Wikipedia articles reflect this.
