What happens in multiplayer Magic when I'm forced to attack and one opponent has Propaganda?
Attacking doesn't target. Attacking doesn't use the stack. There's no such thing as choosing to attack, and then not attacking. A creature either attacks, or it doesn't. What I'm saying is your creature can't attack the player with Propaganda without paying the cost.
You have these options:

Your creature can attack the player with Propaganda (which requires paying {2}), or
your creature can attack a player without Propaganda.
your creature can attack a planeswalker.

As you've said, you are free to choose not to pay the cost imposed by Propaganda[117.3c, 508.1d]. But if you declare to have your creature attack the player with Propaganda, and if you choose not to pay the cost imposed by Propaganda, the attack becomes illegal[508.1c]. Since you can't choose to take an illegal action, the action is rolled back[508.1, 721.1], and you must choose a different player or planeswalker to attack[508.1d].
Only if there are no valid choices (e.g. every opponent has a Propaganda, no opponent has a planeswalker, and you refuse to pay the cost imposed by Propaganda) can you avoid attacking[508.1d]. This is an application of second Golden Rule of Magic, popularly paraphrased as «&quot;can't&quot; trumps &quot;can&quot;»[101.2]


117.3c Activating mana abilities is not mandatory, even if paying a cost is.
508.1. [...] If at any point during the declaration of attackers, the active player is unable to comply with any of the steps listed below, the declaration is illegal; the game returns to the moment before the declaration [...]
508.1c The active player checks each creature they control to see whether it’s affected by any restrictions (effects that say a creature can’t attack, or that it can’t attack unless some condition is met). If any restrictions are being disobeyed, the declaration of attackers is illegal.
508.1d The active player checks each creature they control to see whether it’s affected by any requirements (effects that say a creature attacks if able, or that it attacks if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of attackers is illegal. If a creature can’t attack unless a player pays a cost, that player is not required to pay that cost, even if attacking with that creature would increase the number of requirements being obeyed. If a requirement that says a creature attacks if able during a certain turn refers to a turn with multiple combat phases, the creature attacks if able during each declare attackers step in that turn.
101.2 When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.
721.1. If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. [...]