When logged in as admin (or sometimes for everyone due to an unknown bug), allPlayers also return headless clients. However, it is often undesirable to have them included, for instance if the command is used for a custom player list, and filtering them out of the array is cumbersome and slow.
It'd be great to have an optional parameter to include/exclude headless clients from the output, e.g. "allPlayers true" / "allPlayers false"
as the headless client is also just some player your request is most likely not simple
It is true, HCs are in allPlayers. I don't know whether it is a bug or intended, since HCs are playable by default and have own PC instance. No other command returns HCs BTW.
Saying "Yes, it is extremely simple" without knowing for sure how this is all organised is not very credible.
For starters, not a single nular command in Arma has an alternative syntax, and allPlayers is nular command. So I doubt allPlayers <boolean> is even possible.
Also why can you not simply:
_justplayers = allPlayers - [hc1, hc2,...hc2356];