ANNNND yet MACOs are mentioned in the STO Book The Needs of the Many. Guess your idea they dont exist any longer fails.
However, if we're going to get that nit picky ...
MACOs are not part of starfleet. They're an earth based, human organization. As presented in Enterprise. And the ENT books. And the STO novel, if you read it carefully, supports this as well. None of the MACO's fellow MACOs are aliens in that chapter. They're all still human.
So the suggestion is that MACOs may still exist, but are still an earth based human organization.
That being said ... I'm still very much in favor of increased customization. So if people want to play a MACO and want to be whatever alien/species they like ... I'm all for having that option. Let people run wild. Do whatever they want. And have fun doing it.
Just, you know, if we're going to really start to pick apart that chapter ... all evidence thus far points to MACOs being separate and exclusively human.
There has never, not once, been any canon statement that the MACOs do not exist past the Enterprise era. Just because you don't see them in the other series in no way is definitive that they no longer exist. Using that logic, North America didn't exist before it was discovered because the Europeans couldn't see it. The idea that something doesn't exist because you don't see it isn't just illogical...it's stupid.
Says the presentation of the MACOs in each book so far. Including STO.
Find me an alien MACO in a book. Just one. And I'll concede the point. Until then ...
In that chapter Jake Sisko wonders to himself how the two federation triped races serving in MACO would feel about the use of terms like "trike", "three legged deano", and "kickstand" as terms for the undine. So according to this passage, there are at least two other species besides humans serving in the MACO. They are the triexians and the edosians.
I don't think it's unreasonable to assume that even an organization like the federation that is dedicated to peaceful coexistence would also have a viable ground force to deal with planet based threats. Not every enemy can be fought with a starship. Diplomacy is fine and great, but there will be times when diplomacy fails. If the federation was so naive to bele e diplomacy always worked, then there wouldn't be phasers on starfleet ships. I find it hard to believe that the federation could fight a protracted war on the ground against an enemy like the jem'hadar, the Borg, or the Klingons with just away teams. The federation would need a dedicated ground force and the MACO seems to fit the bill.