Stop of campaigns that pay per posts like Chip Mixer and other mixers are main reasons if I am not wrong.
That wouldn't explain the second chart OP showed, because mixers were banned at the beginning of this year whereas it looks like posts/month have been falling since 2022. That's a very short window of data, and I haven't looked to see what the entire history of the forum shows. Not saying OP cherry picked his time frame, but it's hard to judge when everything before 2022 isn't shown.
Are bounties still a thing? I haven't heard much about them in a while, but they were extremely popular for quite some time. If they went away, I'd expect to see a massive drop. But the members who were in mixer campaigns probably went on to participate in other ones, and I doubt the mixer ban would account for a posting drop as severe as what OP is claiming.
Forum posts don't depend too much on Bitcoin halving.
Lately it seems like the relevance of this current halving has been blown so far out of proportion that I wonder if a lot of members here even know what one is, and if they do, why they expect bitcoin to move up or down or whatever else they figure is a consequence of something written into bitcoin's code long ago.