I've noticed a downward trend, not necessarily "dropped off suddenly". One of the most notable signs I've seen is from the new comments sort.

When I'm interested in seeing what's actually active and where the action is at, one of the things I will do is click to sort by "New Comments" and change my view to Comments. Typically I go to that page, see if any of the headlines or comments catch my attention, then go read or reply.

In the past couple of months, I'm seeing more and more new comments on that first page of results that are 5+ minutes old. When I get to the bottom of the page and click to refresh the results, there are times now when I don't even get a full new set of comments because there haven't been enough new ones to bump the prior comments off the first page.

That didn't used to happen much at all, it was rare enough that it really sticks out when it does happen. Typically, the comments on that first page would be anywhere from seconds to maybe 2 or 3 minutes old and every time I hit refresh (I wasn't spamming the button), I'd have a completely new set of comments to peruse (other than a bug in older versions of Lemmy that would cause some comments to get stuck at the top of that page even when they were significantly older than anything else).

My overall interpretation of this is that it appears there's less commenting, at least during the times of day that I tend to be most active on here. Of course that's not the only possibility. But like OP, I'm noticing a lot fewer posts in those top 6, top 12 filters with lots of comments than I used to see. So, those types of observations do have me thinking things are on a bit of a down trend. It could be a seasonal thing, perhaps a temporary lull.