A few years ago I used to mostly use Series Stats as a way to discover manga to read, but I've noticed in recent years it's almost impossible to use. What exactly changed? It's dominated by Josei, Yaoi + smut manga, and manga with 1 or 2 chapters. Has user trends shifted that much? How do completely new series have such high activity?

If anyone can explain please do and please understand I'm not shaming anyone's genre preferences, just trying to understand why it's almost impossible to use it to find manga I want to read compared to a few years ago.

100 views and no reply for the most broken thing in the site for years, im in the same boat as you, i even tryed to "request change" several time to add filters to the series stats page.
There is no explanation of what it makes a manga to be on top on activity or "real activity"

Check this manga for ex. "Love Gym"

Activity Stats (vs. other series)
Weekly Pos #30 increased(+4)
Monthly Pos #22 decreased(-13)
3 Month Pos #1 increased(+2)
6 Month Pos #3 increased(+5)
Year Pos #11

List Stats
On 43 reading lists
On 33 wish lists
On 6 completed lists
On 1 unfinished lists
On 14 custom lists

Only on 97 users list, with only 9 chapters, no info on translated chapters but its TOP 11 in year activity dosent make any sense.

We have verified the stats with other external sources. Most of the traffic is being driven by Indonesia, and in a lot of cases, people find us via a web search and promptly leave via webtoon links. We have extreme spikes during certain days of the week for certain series. It's unfortunate, but it's a reflection of how our user base has changed over the years.

The other poster makes a good point about filters. Adding tag/genre filters to series stats would make it at least partly functional again.