I’ve always thought the rule was that if any part of the body with which you can legally score (ie not your hand/arm) is beyond the last defender, it’s offside?
As for the point re VAR for offside decisions, it absolutely shouldn’t be used during the game if nothing comes of the chance. We don’t need every inconsequential decision to be reviewed.
However, in cricket if there’s a wicket, they review the bowler’s front foot to see if it’s a “no ball” as standard; you could have something like that - if there’s a goal, you’ve got a man in the studio who automatically checks for offside. You’d need specific rules - you only check for offside from the assist (ie; you can’t go back to a pass which happened fifty seconds ago).
VAR being implemented for things like penalties is tricky though. In rugby and cricket, there are clear phases to a game even in when in open play (each delivery in cricket, or each ruck/maul/breakdown in rugby). In football there aren’t clear phases like that. So, in cricket the technology has a limited amount of things and a limited timeframe it can be checking for - is it a front foot no ball, has it bounced before the fielder catches it, or Hawkeye for lbw.
In rugby, I believe they can ask the tv ref only two specific questions (“I think it’s a try, is there any reason I can’t give a try”, and “try or no try?”), there are clear rules which define what answers the tv ref can give and the tv ref can only go back and review two phases of play (I think). I think the tv ref can also comment if there has been a yellow or red card offence the on field ref hasn’t seen.
In football if a ref asked a tv official “is there any reason I can’t give a penalty?” how far do you go back? Do you check every forward pass since the ball last went out of play for an offside, do you check every ‘fair’ challenge since the last set piece to see if a foul was actually committed but missed, do you go back to the last set piece, or the last time the opposition touched the ball, do you focus only on the specific tackle/foul?
If it is going to stay, there need to be rules somewhat similar to rugby. We need tv officials (not just on field refs watching replays), specific scenarios that can be referred to those tv refs and, ideally, specifically worded questions with specific answers and specific protocols for what the tv refs can and can’t review when coming up with those answers.
For me, it’s too complicated to get right for penalty calls. Goal line technology works and works well. Something such as offside “spot checking” by a man in a studio on all goals scored could also potentially work. Anything else is all just a bit too difficult to get right.