Absolutely, one can disarm a spellcaster of their focus (excluding the cleric or paladin with the painted shield).
It is always a legitimate technique for a PC who is a battle master (with the manouever) and so would be legitimate if an NPC battlemaster did it. If you introduce it to the game as DM, though, you are telling the players that they need to carry multiple focuses.
The optional rule in the DMG allows something to a creature (and, implicitly, to PCs) that is normally something restricted to battlemasters. You know your players, and if it were to be introduced, you should talk to them about adding the rule at least a session before it becomes an NPC tactic. I would never include the rule if a PC were playing a btttlemaster (whether or not they had chosen the manoeuver), but would consider it otherwise (following discussion with the players).
If the PCs wanted to implement the rule (on the fly, or in advance) I would allow it, but it is then a technique that all NPCs have access to as well.