If you've been using SE network for some time, you have surely seen some chat rooms displayed in the sidebar on the right, something like this:
I will add that I consider something like this useful. It promotes chat and displays rooms that had some activity recently.
Main question: Were there some changes to the algorithm that decides whether the box with chat rooms is displayed? Is it no longer displayed on main sites and only on per-site-metas?
A bit more details. As far as I know, the details of the algorithm which decides whether the box with chatrooms is shown or not are not publicly known. And moreover it seems at least to some extent random, so the details might be quite complicated. Still, some observations about how it works can be made from the past experience. I have only seen them when displaying a question. (So, for example, not on the frontpage.) At some point a more experienced user gave me advice that they are typically displayed when there is enough vertical space in the sidebar. So it is good to try questions with many answers. (I do not remember who it was - but they definitely deserve credit for this advice.) I have used this a lot whenever I wanted to see the chatrooms in the sidebar. And - at least until recently - it seemed to work quite reliably, the likelihood to see the rooms in the sidebar was much bigger on such question than in average. (And it still seem to work quite reliably on meta sites.)
Lately I've been noticing that it does not work that well. But I initially dismissed this as probably I just had bad luck. However, after some time I decided to test this in more detail. I have tried a few sites and corresponding metas and tried several questions with many answers. (I have tried at least 10 questions on the main site.) I never saw the chatrooms when trying this on the main site. And it works almost always on the corresponding meta. I will list here the sites I have tried if you want to check for yourself:
- MathOverflow and MathOverflow Meta
- Mathematics and Mathematics Meta
- Academia and Academia Meta
- I will also add that it seems to work fine on this site.
I do not know whether this was discussed also on some other local metas, the first time I heard somebody complaining about this was in this discussion on Mathematics Meta: Chatroom links disappeared. It might be a complete coincidence, but it seems that was around the time when the threads for 2018 community promotion ads were posted on per-site-metas.
EDIT: As far as I can tell, on beta sites it still works the same way it used to be on all sites. I.e., the box with chatrooms is shown on the main site, too. You can try yourself - after clicking on a few posts with enough answers, I relatively quickly stumbled on one where the chat rooms are displayed. (I have tried this on a few beta sites, for example, History, Health, Politics, Sports, etc.)