Halon systems are banned, at the international and federal level, and have been since the 1990s; so, modern DCs don't and can't use them.

Ahh... the Vax War (bit hard to search for it now... gotta search for the first line). VAXen, my children, just don't belong some places.

https://www.hack.org/~mc/texts/vaxen-joke.txt

Realizing that I'm spoiling some of it... but we're here, so some of it is inherently spoiled...

           The fire alarm klaxon went off and the siren warning
           of imminent halon gas release was screaming.  We
           started to panic but the data center manager shouted
           over the din, "Don't worry, the halon system failed
           its acceptance test last week. It's disabled and
           nothing will happen."
 
           He was half right, the primary halon system indeed
           failed to discharge. But the secondary halon system
           observed that the primary had conked and instantly
           did its duty, which was to deal with Dire Disasters.
           It had twice the capacity and six times the discharge
           rate.

They don't have to blow that fast. It can be regulated.

But yes, I've seen low-O2 installations also. These have other issues though, like no protection when humans need to be in there for maintenance. And the need to be very very careful when reducing it again, making sure everyone is out. It happened to my colleague once that this went wrong and while it's not low enough that you'll die, he wasn't exactly feeling great for the next few hours.

Yeah, like the infamous Nasdaq Nordic outage in 2018 due to errant fire suppression gas release.

Maybe because it was in a normal office building? The gas extinguishers might be too hazardous there

Halon is not exactly fun to breathe, either.

We had it at work at a datacenter right beside offices and reception.

You just have to reinforce the ceiling panels and the windows so they don't get blown out, but it's not really an issue otherwise. It was all certified and regularly checked. Water is indeed a bit counterproductive as it's not really much better than fire for the equipment in there.

PS: Halon itself is of course gone but it's other less harmful gases these days.

What also happens is low-oxygen environments (they only raise the O2 when maintenance is being done).