Andrew Heenan wrote:

I know nothing (and could care less) about governmental 'requirements' -
whether they 'required it' or not, it has been superfluous since the
seventies.

I suspect the 1996 change merely codified the day to day reality of the
previous thirty-odd years.

I believe it was more down to database changes. The county, whilst not
essential, is useful for decoding addresses mangled by handwriting. Also
128, IG1 1DD style addresses are vulnerable to people getting the post code
wrong. I've known of several businesses who only discovered the post code
they'd given out for decades is wrong when they tried to generate directions
on multimap.