There was half a fix?
When you clicked custom game, you get to browse what current lobbies were in the MM pool or are actively on.
very few noobs knew this and we collectively kept it a secret. And the problem of mid game joins was now uncommon as very few noobs were smart enough to know. Though some intuitive noobs did but as you would expect, it was rare.
This was a half measure because the main issue still existed. It wasn't fixed. You could still join and troll if you wanted to intentionally do it.