I think people still play divisions with at least some regularity. I've seen a few Rigs communities on the PS4 that seem kind of active as well. I got lucky with the division ranks a few weeks ago when the game was on sale in the US and there were quite a few people online without needing to boost. The EU PSN has it on sale now as well, so it might be a good time to jump in actually.
One human player on each team should be enough, that's how most of my matches went. The remaining positions then were filled out by AI. One time, apparently, the other player quit on me just before the match started and I played the whole match alone against bots - still counted just like normal. That's most likely an exception though.
When boosting divisions, another thing to consider is that each player needs enough points to be eligible for promotion. I've seen a trophy guide suggest that promotions depend just on the number of points gained, but it seemed to me that each division has ten fixed matches that need to be finished with at least a certain total score in order to rank up. Personally I had to do all 50 matches to finally be promoted from division 6. But maybe that was because I sucked 😊
1v1 matchmaking seems to be hard to get without boosting though. I personally haven't found anyone after a combined four or so hours of searching/waiting around. If anyone else needs the Mano a Mano trophy or can help, I'd appreciate that as well! Will gladly return the favor, of course.