This is formally/legally a work break that you’re not allowed to count as work time. If I have a half-hour conversation about a non-work topic, which I sometimes do, it means I’ll need to work half an hour more. At the office it’s effectively at everyone’s discretion how exactly they count it, but on a chat platform it can in principle be tracked if someone spends substantial time on #offtopic.
Legal definitions vary country to country: I wouldn't be so quick to insist on some universal definition. I'm pretty sure you're wrong about US law there - docking someone's pay for "chatting" sounds extremely difficult to defend.
Besides, multi-tasking exists: sometimes I need to let my brain idle on another topic for 15 minutes, because I'm working through something complex, or just wrapped up a project and have a meeting.
Certainly, nowhere I've ever worked has tried enforcing anything like this. I've had plenty of co-workers who made a point of wandering over to socialize for 5-10 minutes every day, which must have easily added up to an hour a day - but they were also the expert that knew exactly where everyone was and who needed to coordinate with who.
I’m not in the US, so that may be right. In my view this is more about how the employee feels about it: I don’t want to get into a dispute whether the half hour a day I spent on the rambling channel counts as work or not. For that it makes a significant difference if people use the channel to discuss their hobbies or whether they discuss work-related ideas. I also don’t want to miss the work-related topics just because I’m not interested in the hobby discussions.
In my country, we are allowed an half an hour break that is not deducible from your work time. It is expected that you need to take breaks in a 8h or 8h24 shift and you are free to decide if you want to take one long one or several shorter ones. Also going into the bathroom is not deducted, even if one day you need 15 minutes to take a proper dump or another day you have stomach issues and need to go more often.
bottom line: YMMV. check your local laws and/or collective agreements.
