I know Blizzard bypasses accuracy in Hail as of gen 4 but not before so I was wondering
Thunder has never had a condition in which it has 0% accuracy, so it can always eventually hit, regardless of the weather. If you're asking if it can bypass accuracy, then:
When used during rain, Thunder bypasses accuracy checks to always hit, unless the target is in the semi-invulnerable turn of a move such as Dig or Dive.
Yes, from Gen II onwards.
Thunder (Bulbapedia)