This was, I believe, later extended to permit 4 digit year numbers and to
permit both an offset and a parenthesized zone name for the time zone.
RFC 1123 made 4 digit years a "SHOULD"-level requirement, although the IETF
area chairs that cover E-Mail have been quite vocal that 4 digit years are now
to be considered mandatory.
The timezone offset has always been legal. What changed is that RFC 1123
deprecated the use of American timezone names, completely barred the use of
the US Military timezones, and recommended using the offsets exclusively.
Putting the zone name into parentheses is relatively recent, but was always
legal: parentheses are the comment delimiters, and whatever is inside them is