It is technically against the rules as it is collusion. But they have to prove it. You should never openly agree to not play for a side pot. Simply check the pot down.
Late in a tournament the money jumps are worth more than taking down a pot or getting more money in a pot. The rest of the table benefits if Player A goes out.
If A is all in and B calls it would be legal for C to over-bet to fold out B and isolate on A. That would commonly be considered bad form. The optimal money play is the just check it down and maximize the chances of taking out Player A. If A is small stacked anyone not small stacked should call and check it down. If you call in the dark you have made your intentions to check it down clear. Some players will even announce "I call in the dark" and pretty much everyone knows that means let's collude but you will never get called for it. It is an accepted practice as long as you are discrete. Check it down and value raise the river is definitely bad form.
I saw a play where B and C called pre flop and C bet the flop to get more money in the pot. B folded. C lost to A but B would have sucked out to take the pot. Instead of taking out Player A he got tripled up. B tabled his cards to let C know that he messed up.
Generally, any form of agreement between two players that harm others is against the rules. That includes checking a hand down. In fact, most casinos DO NOT allow game-play talk in a pot with more than 2 players to prevent some sort of communication between the two.
Imagine a situation where you have a 5h 6h, your opponent has Ah, Kh, flop is 4h, 8h, As. You are losing pretty badly cause you are basically a gutshot draw (4 outs). However, what if your friend has 4s 8s? Your friend is currently winning but the opponent still have two Ace, three Kings, seven hearts... That's 12 outs per street (almost a 50/50 winrate with both streets).
Ideally, in a competitive play, your opponent and friend should raise you out and isolate the AK vs two Pairs cause your draw is far too weak to call any real bet. However, in collision play, since you and your friend are friends... You might want to slow play and make sure you are both in the game. That way your friend wins 50% of the time and even if he doesn't, you might still win with a royal flush or straight. If you share profits, the two of you will net more gains with slowplaying.
It's EXACTLY this kind of collusion that concerns players so generally any kind of communication that could lead to collusion is disencouraged. Obviously, you checking blind to "indicate" you want to slow play is ok, but if you talk to your friend and signal what you might have is not.
The same exact concept applies here. Any sort of communication leading to a "work together" against another player is against the rules. However, most seasoned players are smart enough to recognize those kind of play and would do it naturally.