As far as AL is concerned, there is no rule for "Disarming" outside the the Battlemaster maneuver. As this is effectively "a situation not covered by the rules" expect table variation. Some DMs will flat-out say you cannot disarm creatures unless you are a battlemaster with the disarm maneuver; others might allow a lesser kind of disarm. I myself have been known to allow someone to try and disarm an enemy as an opposed check (Athletics vs. Athletics or Acrobatics) as a special attack (similar to grapple/shove). I tend to grant advantage to creatures wielding a two-handed weapon, and disadvantage to their check for creatures/PCs wielding a light weapon.
I have also used Opposed Attack roll (with weapon) vs. Attack Roll (with weapon) for similar results.
What I don't allow however, is for non-Battlemasters to have the skill to grab the enemies weapon or to throw it away from the creature (it always lands at the enemies feet, but there is nothing stopping the PC from picking it up as an interaction with an object.
Thing is, I also don't allow PCs to do this repeatedly. Only Battlemasters can disarm creatures at-will (within the limits of their superiority dice). All other creatures are limited to one such attack per short or long rest (simulating their lack of skill, as well as the difficulty in finding an appropriate opportunity). Even then, I only allow this if a PC specifically asks if they can disarm. Otherwise, I don't even mention it as an option.
Also, I was not aware that there was an optional disarm rule in the DMG. In light of this - I will probably stop allowing PCs to disarm enemies (excluding BM fighters) - as I wouldn't want players to think I was adapting an optional rule, and modifying it.
