The most recent update for World of Warcraft Classic has the Burning Crusade Battlegrounds welcoming large pre-made groups again.
The update made it so that grouped players will be matched against other groups of similar sizes, and the matchmaking tool will put raid groups of six or more players against a group of six or more players. This should see things get a little more fairly distributed. The matchmaker will also fill in undersized groups with individual players or smaller groups to bring your side up to a more competitive size. it will also make sure that groups of five or fewer players that want to queue will also be matched to groups of five or fewer players. So if the population has an issue at the time you're looking to tear it up in a raid, you'll still be able to get in and get involved without serious imbalances. This should help groups and individuals alike to get in there faster and at a somewhat realistically competitive playing field.
If you do queue up alone or in a smaller party it doesn't mean you won't be matched against a group without pre-mades, the changes in effect now generally just guarantee the group size should be roughly equivalent. Of course, you should still gear up and prepare accordingly, but the groups and pre-mades back in Battlegrounds on Classic should help with population, cohesion, and still make room to balance out lower population times and let everyone get back into the loot hunt.