If i'm not mistaken, the very last graph showing posts by month shows a significant decreasing over the summer, or at least specifically july (since august is not over yet). It says June is around 12M posts and July around 9M. August seems already over 9M and we're only 2/3 in, so maybe there was some drop around July for some reason, which could explain OP's feeling.

As for the reason, i'm not sure, maybe a combination of factors like some server blocking access from UK with the latest privacy bullshit, some server having signficant downtime (i think slrpnk.net had a problem with servers locking themselves up while tech admin was abroad), and maybe non-Lemmy factors like holidays (though they don't seem to impact previous years stats).