All forms of mirroring commands are now against EULA, according to the latest update of the agreement from Blizzard. The last update happened in Nov. 2020, when Blizzard added input broadcasting software to their restriction list.
With the latest update, outside of input broadcasting software, other types of hardware and software that mimics a keypress across multiple accounts are prohibited and could lead to account suspensions. These actions allowed players to multibox in World of Warcraft, using the same movement or ability across different characters on different accounts at the same time. There have been multiple examples in the past of players using this to quickly delete entire groups of enemies with several characters casting at the same time.
Those who want to keep multiboxing will have to adapt to more manual workarounds, such as coupling multiple keyboards and pressing the keys in various keyboards at the same time to continue their current playstyle. While some players attempted to evade the last time the policy was updated, Blizzard issued warnings to all of them, before notifying players that the warnings will escalate into account suspensions if the actions continue, even up to a permanent closure of the WoW account.
Multiboxing still isn’t against the EULA, as players are able acquire as many subscriptions as they want and play them with different clients. But Blizzard’s stance seems clear after their latest action—mirroring commands to multibox aren’t welcome any longer.